Again, no direct experience with retreats in the Amazon, but I do have some experience with a variety of ceremonies and rituals in North America, so I'll attempt to answer a few of your questions based on that experience.

I wouldn't be too concerned about dosage, most providers are happy to pour you another cup if you ask for it.

Having only a couple or a few sessions in a week is generally deliberately done to allow time for proper integration of the experience before immediately jumping in again. It allows you to reflect, have a chance to ask questions, and ensure you absorb the lessons.

Similar to the above, it may be more productive to work more intentionally with a single teacher than to work more peripherally with multiple teachers. Graduate dissertation in ayahausca may have more depth to offer than auditing aya 101, huachuma 101, and sapo 101. If the goal is to work with the medicine on it's home turf, it may be worth a separate trip to the mountains.

Certainly smaller groups allow for more individualized attention and would be preferable to me, but larger groups are more economical, and most places should have sufficient assistance available to ensure everyone has enough support. You will have to deal with that many other people's "stuff" arising during the ceremony though. That's just the nature of doing the group work.

Lastly, don't let budget be the main deciding factor here, read reviews and take them into consideration, but above all else listen to your gut and walk away from anything that doesn't feel right. Authentic integrity shines through, make that the basis for your decision.