I would be okay with 64 bit (PowerPC G5), but Linux or rather BSD is a step I can take if everything fails on MacOS.
For one thing, it will be inconvenient for practical reasons (rebooting into other OS just for the sake of running R). From another side, making it work on MacOS will probably benefit someone else who faces a similar issue. It is sad that many developers give up on PowerPC, since G5 ones are pretty capable even nowadays.
(I have a backup route with Julia instead of R, but perhaps all the same issues with Stan will arise in that case too.)