Is there a gba emulator app like this?

You have mGBA or injects, other than that, no.

 

Try GBARunner2, it runs in NDS mode but IMO it has better speeds than mGBA since it is an hypervisor rather than an emulator (only sound and some buffer instructions are emulated), and you can change roms like you said in it.

 

gpsp is not crashing anymore these days, but you need a n3ds to make it run fullspeed.
it's also infinitely better than mgba

if you're on an o3ds, vc injections are about your only real option.

that said, i haven't looked at gbarunner in a long while, last time i tried that, it was full of graphical glitches and sound issues, but maybe its better these days

 

Well. ya aint getting that now are you. You have your options infront of you. No ones going out of their way to make something like that for one person. Be happy with what you got.

 

Basically any form of emulation that is not VC is automatically inferior no matter what, since GBA VC on the 3ds is literally a GBA game being run on native hardware (The 3ds acts as a real GBA at hardware level). The only option would be that someone reverses the AGB_FIRM to give it SD Card access, and that Firm is pretty obscure at this moment so...

 

well, they're right. injection is the only thing to give you just about the most accurate gba experience.
gpsp is more than good enough on new 3ds, but i'm sure theres a few games with graphical issues and sound isn't 100% perfect. but not to the point where most people would even mind.

mgba doesn't run well on any 3ds because accuracy just isn't within the systems capabilities.

and i see gbarunner still has it's issues.

but those are all the options

 

gPSP works with most 2D games but it's largely broken with true 3D games (not including the "mode 7" stuff like mario kart/f-zero/krazy races, those work fine.) I suspect it doesn't support the per-pixel graphics modes well if at all.

Still there's tons of great 2D games and gpsp is a good way to emulate most of them.

 

Obscure? https://gbatemp.net/threads/what-can-i-put-on-my-3ds-to-use-in-luma-chainloader.564199/#post-9044678

 

Oh shit. This is cool: https://github.com/profi200/open_agb_firm
It's not ready yet but the last commit was five days ago. So it's still being developed.