I agree with what you call disconnection. There are many who forfeit the traditional ways. Then again, it is a lot easier to just serve the medicine, than to actually do a ritual and to work on and with those who came looking for help. Then again, the ayahuasca-tourism is vital for many communities to stay connected with modern days society. There are tribes that have retreated back into the depths of the jungle, but for most this is not an option. Selling and serving ayahuasca is a great way to both teach a way of living, but also allowing interaction with our capitalistic culture.

There is nothing lost, but not everything is meant for the public. There was a time, when Shamans showed much more of our skills. But there was also a time when people hunted tribes down for their gold. There was also a time when we burned witches.
A shaman is typically an advisor - our connection with the other side helps to see and listen. In shamanic tradition, there is a big element of healing and guidance. But, first and foremost, a shaman is connected to an aspect of reality that is not meant to be described with words.

Then again, many call themselves Shaman, although they never finished their training.

Dosage is somewhat secondary and the required amount often varies, depending on the ceremony. "Measuring" Ayahuasca is additionally complicated by the spiritual aspect - every brew needs intention to work properly. On a side note: traditionally, only the shaman drinks.

As for comparing the data: usually, ayahuasca comes with one or multiple purges of some kind. The only real differentiation that seems viable is before/after final purge. Either you are looking for guidance/healing or you are not - at this point, ayahuasca becomes interesting in a recreational setting.

Considering your side note: People feeling a major void or even drifting towards nihilism usually happens if they journey without a guide. This is one reason, why both pre- and postceremonial integration are critical. It can be difficult if not impossible to recover from an egodeath. Sometimes people want their bodies to follow their minds.

As for your last note: While there are some cultures that brew it thick like a vegetable soup, others actually prefer a thin and nasty tea. Although the nastiness fades once one has shed their burdens. it becomes rather tasty after that point.