Man OP you lucked out, GB/GBC RPGs and games are my specialty. I've been collecting them for a long time now so I can give you a few more recommendations. And some romhacks too.
>Magi Nation
Quirky 90s RPG that came out on the coattails of Pokemon. Full of witty dialogue and 90s-era coolness. God-tier chiptunes as well. Supposed to be part of a trading card game franchise or something but I never learned more about it.
>Final Fantasy Adventure
This is what became Sword of Mana in 2004 on the GBA, simpler since its a gameboy game but still fun. It was given the Final Fantasy title for brand recognition.
>Final Fantasy Legends
There's like 3 of these games but they're actually the beginning of the Gaia series, like FF Adventures it was to help with marketability. Turn based RPG where you can change the race of your party members. Kinda SMT-ish
>Another Bible
This is actually an SMT game, it was the only one of the dozen gameboy games to get an English release. I'm sure you can find the others if you look for translation patches.
>Survival Kids
Survival RPG where you're trying to help a shipwrecked 10 year old survive. Pretty challenging as it doesn't hold your hand.
>Metal Walker
Kinda odd RPG where you customize robots and then..Bonce them off each other. Battle mechanics are odd yet fun and the arts pretty great.
>Alone in the Dark
Near the end of the GBC's life there were a bunch of psuedo-3D games that came out that are incredibly granulated and difficult to figure out whats going on. This is one of them. None theless this is an RPG and a pretty good demonstration of what the hardware was capable of.
>Monster Rancher Battle Card
Pokemon TCG-like with an interesting mechanic and horribly Engrish translations. Guts don't mean shit
>Yu-Gi-Oh
Another TCG game. I always liked portable ones so there you go
>Pokemon TCG 1&2
You probably already know about these. The second one never got an English release but there's translation patches out there.
>Bomerman Max Blue Champion/Red Challenger
These were bomberman games that had adventure-rpg elements. The GBA games and GC games took on a lot of these elements down the line. There's no difference between the versions beyond who you control, bomberman or the BDSM edgelord.
>Robopon Sun/Moon
Another pokemon-esque RPG with robots, only you actually fight them unlike Metal Walker.
>Warlocked
This is to my knowledge the only RTS on the GBC. Try it if that interests you but if you've played RTSs on genesis or any older systems you should know how they'll play.
>Warriors of Might and Magic
This is a port of the action-RPG PS2 game I can't honestly recommend this one because my cartridge has never worked in any of my systems
>Crystalis
Shitty port of a NES game but still a fun adventure RPG.
>Shadowgate Classic
Point-and-Click Adventure game. Its challenging.
>Lil Monsters
Probably the worst quality pokemon ripoff on the gameboy. You're welcome to try it but don't expect much.
>Legend of Zelda games
There's 3 of them: Link's Awakening (DX is the best version) and Oracles of Ages/Seasons. OoA/OoS have some cool cross over features but idk if you can make them work on an emulator.
>Dragon Warror Monsters 1&2
I can't recommend these enough, OP. Honestly superior to the Pokemon games to this day. The breeding system is so intricate and deep that you will probably never 100% the games. Also Dragon Warriors 1-3 are good too.
And some Non-RPG recommendations:
>Gargoyles
Ghouls n Goblins spinoff with a weird bit of platforming and topdown adventuring. Still really fun.
>Balloon Kid
Balloon Fight spin off that's a platformer. Pretty intense.
>MoleMania
Kinda plays like Goof Troop if you've ever seen that one. Top-Down zelda-esque game with puzzle elements.
>Kirby's Dreamland/Kirby Pinballland
This is where kirby started out. I'm a huge fan of pinball games so I'll probably always recommend them.
As for romhacks, Pokemon Brown is one of the first completed romhacks (I think 2003 or so), so it'd be worth looking into. I think its based of either Pokemon Red or Yellow.
Hope that helps OP, happy playing.