Now this is clever, no need to hack THEC64 Maxi with RetroArch to play other emulators, just put a RPI3 inside it then install RetroArch in the RPI3 or VICE and switch between them..
PCUAE will not damage your THEC64 Mini/Maxi, it don't touch the firmware and don't change anything in it and will not void your warranty, it non intrusive, it dose not touch the firmware.
REMEMBER, If you don't understand anything, you can ask me.. or read the posts in the PCUAE thread.