The effort to support architectures is now minimal with AI assistance. I made a hobby architecture (based on Intel, but with some changes; I was making an "alternate history" as if a few decisions in the past had been different) and it was pretty much trivial to spit out support not just in gcc and llvm, but I also, for fun, made WATCOM backends and a few other things.

With that said, RISC-V is a nice baseline for designing another architecture. Start with RISC-V, and go from there.