Hey there! Welcome to the Stan forum! Sorry for the delayed answer, somtimes the numer ob brms question overwhelms the the number of brms experts on here, so some topics slip through. That being said, I’m not actually a brms expert.
I had asked a similar question some time ago and it seems that this is not supported (yet? or maybe it is by now? @paul.buerkner would know).
If you are willing to work in Stan, you can get this to work. You could start by looking at whatever make_stancode(bf(y ~ 1), dat, family = mix, prior = prior) gives you.
Oh and btw, you probably want to run less iterations (HMC/NUTS really doesn’t need those super long chains), but more chains (default is 4) to see if chains are actually converging. (If your still in model building phase, running low number of iterations on a single chain is fine, though.)
Cheers,
Max