A ROM browser is not planned, since GBARunner3 is meant to be used with frontends like TwilightMenu++ and another one that is in current development, there's also hbmenu which already supports argv. As for preferring a ROM browser over infinitely better game compatibility is beyond me. Btw, the project is open source, you are free to add the feature any time if you want.
It has been fixed since November 2023
Some 3D game may run slightly faster if their framerate isn't hardcoded. You can try Smashing Drive for example, which runs faster in DSi consoles compared to real GBA hardware.