There’s a lot of great stuff but the console leans japanese heavy, and hugely leveraged Bandai’s anime connections for its titles, so if you’re not into early 2000s anime (one of the worst eras for the medium I’d say) you might be put off by that as well.
The other unfortunate thing is these days all the stuff that is at least somewhat english friendly is expensive, precisely for reasons like you’re mentioning above.
Anyway, here’s a list of good games.
Makaimura - it’s got special vertical stages and is basically an entirely new makaimura… not sure who actually made it but it’s pretty cool.
Clock Tower - one of my favorite versions of the game - it has a few extras vs the snes version, which eventually got incorporated into the PS1 version which is kind of fun. The greyscale really adds to it in my opinion.
Dicing Knight. An ARPG that came from the wonderwitch competition. It’s beyond anyone’s reach price-wise but is good.
Judgment Silversword. Legitimately excellent vertically scrolling shooter (there should’ve been more of these) with unlocks over time. It’s very much a 1cc type of thing and definitely a game you can get better at.
Rockman and Forte. An okay megaman game, but I only really like megaman 8 so what do I know.
Rockman exe. As above, but in color.
One Piece Battle Colosseum. A smash bros-like but with moves that take a lot longer, so it’s a bit slower paced. Also no knocking people of the edge of the screen.
Guilty Gear Petit 1 and 2. Tiny guilty gears, pretty good.
Pocket Fighter. It’s pocket fighter, but black and white. works fine, but the controller isn’t great for fighting games if you’ve played stuff on an NGP.
Tane wo maku tori. It’s a puzzle game about guiding water droplets to make plants grow. I like it! It’s kind of underappreciated and it’s cheap.
Gunpey. Good puzzle game. For whatever reason I like the original Gunpey more than Gunpey EX, the color one. Tarepanda Gunpey is also fine.
Kaze no Klonoa. Another cool game where you switch between vertical and horizontal. It’s a pretty decent Klonoa!
Rhyme Rider Kerorikan. A weird diagonally-scrolling game from nanaon-sha, which I quite like. I played this one a bunch back in the day.
Final Lap 2000. This is one of the few pseudo 3D games on wonderswan. It’s not the very best but it’s fun to see them try that stuff on here.
Magical Drop/puzzle bobble/puyo 2. It’s those games, but they’re on here now!
Macross: True Love Song. A tactics RPG/dating sim. You need japanese language to appreciate it but it is a good mix of genres.
Rainbow Islands. Vertical, original rainbow islands game. also expensive.
Mister Driller. It’s mister driller!
Riviera. This got ported to other stuff but it remains at its coolest on the wonderswan in my opinion. Original non-licensed wonderswan RPGs are tough to come by anyway.
Star Hearts. Cheap action RPG but you do need some japanese knowledge.
Uhhhh that’s what I can think of right now!