Please try placing the core and ROM files inside ROMBundler/Resource.
I tested it on my old Intel Mac running Catalina, and it worked well. Take a look at the screen recording for reference:
https://github.com/user-attachments/assets/1e174ded-161f-4d79-af57-8cece7c6e2d5
Or PR https://github.com/kivutar/rombundler/pull/43
Core+ROM
apple/osx/x86_64/latest/genesis_plus_gx_libretro.dylib.zip
https://kivutaro.itch.io/shrine-maiden-shizuka
Let us know the result