> do you think GCC’s m68k backend (to pick an arbitrary one) has been as battle-tested as their AArch64 one

m68k might be a bad example to pick. I was using gcc to target m68k on netbsd in the mid 1990s. It's very battle tested.

Also, don't forget that m68k used to be in all of the macs that Apple sold at one point before they switched to powerpc (before switching to x86 and the current arm chips). You could use gcc (with mpw's libs and headers) on pre-osx (e.g. system 7) m68k macs.