Trades evolutions done by level, all at Lv 50. I also implemented your earlier typing ideas.
New update coming in a minute, as I just figured out how to get pikachu to evolve by happiness, one of my major goals! It went faster than I expected, being that I'm a novice. That just leaves the annoying walking sprite issue, which honestly I'm not sure I can fix at my novice level, although thankfully Nintendo's Pikachu sprite looks pretty good. You can still force evolve pikachu by thunder stone instead if you really want, for those people who cruelly box their pikachu and hate raising its happiness.
Moon Stone being available in shops is a good idea, since it's a 151 hack. I'd forgotten it can be hard to find enough moon stones to evolve all the mons, thanks.
Yeah, I haven't looked at altering TMs yet, but that should be simple. I think I might have already altered Venusaur to have Sludge (like I did a number of poisons) but I'm not sure, need to double check as I forgot. Coulda sworn it already had earthquake but I guess that must be later gen.
I have difficulty seeing Starmie as part electric, but it would be different, give a great new typing (I always loved Lanturn) and definitely make the Misty battle harder, heh; I know putting Thunder on Starmie is a very popular moveset, so it wouldn't be that different in some ways. Although Starmie is one of my favorite psychics, won Pokemon Stadium with her. Losing that STAB would hurt a bit, although STAB thunder on Starmie is nothing to sniff at either.
I think it might also be interesting to have Golduck Water/Dark instead as dark types often learn psychic moves too? Or would that just be counter-intuitive?
I've also made Golem pure rock in latest update; I believe this matches the Alolan form and would give it less weaknesses, although Geodude can stay part ground to add more difficulty against your starter Pikachu.
While this is the first time this has been implemented in Yellow and any 151 hack to my knowledge, I have to say I think I've seen similar ideas before, so I can't claim to be that brilliant, sorry. It is a shame more hacks don't use it, though.