Well how about that… setting pm.sample(..., mp_ctx='spawn') lets my more complicated model B1 run perfectly - thanks @jessegrabowski for the idea
There’s a couple of URLs in the comments inside pymc.sampling.parallel.ParallelSampler linking to Github chats from 2020 and 2022, and in those a further link to PR Set start method to fork for MacOs ARM devices by bchen93 · Pull Request #6218 · pymc-devs/pymc · GitHub
Perhaps that code can be undone now that we’re a few years down the line?