I think setting
nixpkgs.config.retroarch = {
enableDolphin = true;
enableMGBA = true;
enableMAME = true;
};
etc. should work. You can find possible options in the nixpkgs repo in the pkgs/top-level/all-packages.nix
file when you search for retroarch
.
Last time I tried it I think I needed to download the cores in retroarch again, but I think they didn’t all work when I didn’t enable them via the options above. I guess you have to experiment a bit what works and what doesn’t.