As for sprites, that's all ripped and ready to go. Stats and movepools can be banged out in about week. But as of yet, no one knows how to program the branching evolution method I want. :( So the project's on indefinite hiatus I'm afraid.
I think it may be annoying to have to swap around held items like that, wonder if you can do something less cumbersome but with the same idea. I enjoy your current balance setup immensely and the far more slots to add more evolutions inbetween would be amazing as it is already very engaging and flexible. I'd unironically say things like emerald's pichimon letting you get all the lategame water types like if you can get a relicanth early fixes the replayability and choice issues nintendo has while not just selecting a different early spread.
If there's a way to do something this intricate, even if simplified, it'd be a natural massive evolution. Restrictions bring us maybe better ideas in the end, even if on paper it seems weird, but as a player or how it plays out could be amazing! There's 'evolve when knowing a certain move' and other weird ways like in certain locations maybe generalized to caves or mountains given the region or just wormadam code, genders, stats see pokemon infinite botamon for a direct look, legitimately just a rng check using personality values like wurmple, you can get crazy with it and cut off anything op or too weird early, one step further from the current hacks and maybe one day you can do a hgss or other bigger hack for your crazy big open ended idea?
I personally in crystal thought i changed lines from mokumon's to coronamon's and kept smokescreen on him as the benefit from doing so, you can wire it up so ending up with moves you shouldn't have like the story games, like your previous evolutions change your options and the journey together works out great. Even if my guess was WRONG.
Also i imagine it'd be weird to accidently get the same mega at the end or want to always go to an ultra if an option, potentially having the open-endedness actually LIMIT what players want and maybe the balancing choices, see cyber sleuth for that issue. Overall a bit of style, texture changes like in Emerald go very far for feeling like an improved version of the original systems. Overall reaching this mass of 'my great idea i want but the game engine cant support it' is a huge milestone in any developer's life, ive been there too!