I had a lot of fun with Pokemon Xenoverse. I think it ticks a lot of your boxes.

I know you said you’re looking for something harder and more mature, but I cant help but recommend Pokémon Bushido, and Pokémon Xenoverse. Both excellent games imo.

Pokemon Eternity https://discord.gg/TZWNXrxK

More mature, traditional mmorpg style questing, real time battles against NPC's, wild pokemon, and other trainers.

#1 Pokemon Vanguard is probably the one I'll play if I ever want to play a fangame again https://reliccastle.com/threads/3066/ Fix for any lag from the developer: "The engine the game is built with has a memory leak issue I'm not currently able to fix, if the game starts to lag, save and hit f12 to soft reset and you'll be good for a bit."

#2 Pokemon Eon Guardians is good except it has issues with lag https://reliccastle.com/threads/5445/ This is supposedly the fix for the lag

#3 Another that I know of is Floral Tempus https://www.rebornevo.com/forums/topic/38862-pok%C3%A9mon-floral-tempus-ex-v194-released-12-gyms-elite-4champion/

If you want something familiar but also completely different, go with Pokemon Emerald Rogue. It does what it sounds like and turns the game into a roguelike.

my recommendation is Pokémon Unbound. really enjoyed my experience with that one!

Pokemon Theta Emerald Renev is one of my favourites. It's basically Emerald, but with updated mechanics, attacks, and Pokemon, so you can have up to Gen 7 Pokemon in your team. The Pokemon list also means you can complete the Pokedex mostly on your own, and other cool features. Definitely recommend checking it out.