I have some success in proceeding toward building rstan, though V8 failed at the moment: Override dependency on V8 or make V8 build on PowerPC Mac? (R 4.1.2)
If that is sorted and I am ready to build rstan itself, all the same issues as with CmdStan will arise, right? I am thinking how to minimize “dependency hell”, since fixing every failing component is usually a non-trivial task in its own right.