Binding Coil, Crystal Tower, Void Ark, Azys Lla... yeah, Amon did A LOT.
A takeaway from Shadowbringers, though, which applies easily to Endwalker, is that shattered Ascians can easily carve out their own path. You have. Gaia was starting to despite another one who loved her - the mind stuck in stasis as Eden - trying to get her to join him (PS - in the Savage what-if,
he succeeds and she becomes the final boss of E12S to the tune of The Extreme from FF8
)
There's also precedence that the more you do the body hopping thing, the loonier you become over time (see: Lahabrea). Heck, there's even some further fuel for the insanity theory in Zenos himself, what with the person-who-should-be-capital-D-Dead still hounding you. So yes, Fandingus was Amon once, but no telling who or what he was before that (but after the Shattering.) Also of note is that the body of the possessed did not approve - you've been in the Aitiascope, you saw just how livid Baby Bowlcut was afterwards- and for once his hatred wasnt primarily directed at us. (As an aside, I wonder how Ardbert-in-us reacts to Elidibus-in-Ardbert, or if the First has it's own flavor of lifestream where a part of either of them are.)