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There's a culture of master/apprentice relationships, where each high-ranking witch has a few protégés. The high-ranking witch encourages competition between her apprentices, who are eager to gain her favor.

However, this works hierarchically. The high-ranking witch is one of a few apprentices of an older, even more powerful witch. Members are held accountable for the failures of their subordinates, so competition between high-ranking witches gives them an incentive to make their apprentices as effective as possible, as this reflects well for them.

Since it takes a long time to train, it would be a wasted time investment to throw away an intern who's already been trained in the basic stuff their master doesn't want to teach some new guy all over again.

That's a solution that doesn't require any magic, just a few tweaks to the workplace culture. The organization can be expanded as a pyramid without breaking this effect.

You could turn the system so that the lowest-stage witches need a power investment from the witch higher up. Say, they don't come to the coven themselves, but instead more experienced witches need to find new recruits and become their patrons.

In order for these new witches to be able to do at least something, and also to ensure their loyalty, elder witches need to invest them with the part of their power. If the young witch dies, the power is lost irretrievably.

This way, there's a balance between enlisting young helpers and keeping powers for oneself. Also, there is a necessity to protect them and keep them alive - even if elder witch plans to sacrifice her helpers later in a dark ritual to retrieve the power back.

They can advance when they realize the occult they are studying is a lie.

Western society does like to promote this idea: "one studies for their first part of their life, producing nothing of worth, until they graduate, and then they stop learning and go do things." Every aspect of that phrase is false, but it is the rationale we instill in students in order to get them to suck it up through college. This process becomes absurd when one starts to look at learning phases which extend the lifespan of the learner.

The witches move up out of the intern status once they realize this.

Fundamentally, the older witches better understand a fundamental problem: if they die without ascending to goddesshood, everything they learned and taught disappears with them. Witches which don't understand this tend to lead covens that cease to exist soon thereafter. Thus, from evolution, we can expect the successful covens work around this.

There's plenty of ways to make this happen, but my favorite is to make it so that, once you have learned all of the occult (a lifetime of work), you can synthesize this into an obvious conclusion: the path described in the training doesn't lead you to any lasting change -- just suffering. And yet, there's something cloying about the end-state the elder witches describe. It looks like it should almost work. The "correct" path may be tremendously close. It's just not the specific path taught in witch school.

In such a system, the eldest witches would certainly be plying the universe for its secrets in the best way they know how. But they're just one witch. They can't try everything. Thus, they should rely somewhat on the apprentice witches. If an apprentice witch finds something small that the elder witch missed during her training, she may be able to build upon this small thing and bring the entire coven closer to the right answer.

I'd recommend there be an interesting phrasing in the book about how only a child can lead to the true path to goddesshood. Many witches may mistakenly translate that into child-sacrifice, which would do well to keep the evil flavor you appear to seek alive in your covens, but perhaps the eldest witches figured it out. Only the child-like mind can possibly find the subtle overlooked thing which makes it all work.

Think like that for a while, and then look at social constructs you consider to be "respectable and mature." Look at the father who volunteers at the YMCA to help keep kids off the streets, or the martial arts teacher taking his time to improve his students' lives. Or perhaps it's the minister or priest who truly strives for greatness for their entire congregation (and maybe, just maybe, the entire world). Look at how they act, and borrow that for your witch covens.

They must feel their underlings' pain

Part of the initiation involves a sharing between higher and lower and peers, after which each human will forever feel some of the (physical) pain of the others.

This means that some coven members will encourage someone else's underlings (with whom they have not shared) to be fodder and get killed, while protecting her own underlings and the life of her peer (whom she has shared with). This encourages intrigue within the group while seeming to cooperate.

It also means that initial screening will weed out folks prone to headaches, chronic pain, or severe cramps, regardless of how good a witch they might make.

No "employee" that takes that much training would ever treated as trivial. You are probably picturing something like a gang where you lose one of your street dealers, and you just go out and recruit some other kid off the streets to take his place a few days later. In fact, this being a criminal organisation makes the replacement of intelligent subordinates even that much harder, because you need to find someone both desperate enough to break the law, and competent enough to master what you are teaching. Your senior members NEED new acrolites to grow their influence, and if they keep leaving or dying after 1-5 years, then the organization will crumble.

The Solution

Recruit separate thugs for the sake of being thugs. Your coven has enough power and wealth to attract the normal desperate masses that would join any other criminal organization; so, 95% of the people you recruit will never be in it for the magic. They just have a job to do, and you pay them well for it, but when you find someone with the intellect to make more of themselves, you choose them to be an apprentice and start the long task of teaching them the ways of the arcane. From that day on, they would likely be kept as insulated as possible from the daily dangers of the illegal aspects of this organization until they are strong enough to stand on their own abilities as a formidable underboss.

Rarity. Not everyone has the potential for the occult; in fact, just finding an intern with any promise may take a few years. If your ritual takes five talented witches at a minimum, then a witch dying will be a serious blow to the coven unless they have backups, and by the time one has more than four backups, they'll probably have left to form their own coven (they wouldn't be in this business if they weren't a bit distrusting of larger groups and willing to tamper with things they don't fully understand).

A variant on the hierarchy idea.

Casting spells is a dangerous task. The energy needed is unstable and can lash out in any direction, but it can especially backlash on the caster.

So a witch needs other witches to focus the energy, as a supporting structure.

The stronger the witch and the stronger the supporting witches are, the safer the spells are, the mightier the spells that can be cast.

The lowest in the hierachy need to be trusted, so they don't turn against those above them.
Otherwise casting spells can be life threatening for a witch.

A goddess witch with the support of all lower witches can then safely perform godlike spells.

This might be the first problem I've seen that could actually be made better by a pyramid scheme.

In order to exert this level of control over world events, your witches use spells that require more magical energy than any single human can possess. Witches will mystically bind themselves to their subordinates, which accelerates their ability to learn and allows the witch to use some percentage of each subordinate's magical energy (direct subordinates, plus anyone else all the way down the tree). These "commissions" accumulate as you move up the hierarchy, leading to big "bonuses" to those at the top.

For example (adjust your own numbers as needed), assume each human has 100 "points" of magical energy, the binding bonus is 5%, and each witch had 5 subordinates. The lowest level of the hierarchy would have only their 100 points. Those on the second level would have 125 points (100 + 5% * 5 * 100). On the third, 256 (100 + 5% * 5 *(125 + 5 * 100)). Those at the top of a large tree can leverage their bound subordinate's power to accomplish nearly superhuman feats.

This creates a strong incentive to keep your tree full and healthy. Wasting someone at the bottom level means you lose their power commission. Commissions compound as you go up the tree, so a loss at any level will be felt. Instead, the incentive is to protect those in your organization and keep them invested in training, increasing their own power, and eventually bringing in their own subordinates.

It would happen naturally.

Coven members and apprentices require investment by the witch to teach them, or recursively from others the witch has taught.

Coddler witches, those that don't cause their recruits to die en-mass, end up producing more witches per unit effort.

Over time, the Brutal witches will end up being outnumbered by those that don't waste precious resources, unless they use those resources in turn by attacking the Coddler witches (or, at least their recruits).

Coddler witches can have people die, but you aim to have people who (a) are cheap, and (b) are effective for their costs. Someone who has done 2 decades of abstract magic research isn't much more effective than a well fed peasant.