Sys-audio and bluetooth - how safe?

Yes, as far as I see it.

Not that I’m aware of

Usual regarding bluetooth.

It depends on your neighbors, I’d say.

As far as I see it not bigger than any audio in dom0 to any other qube.

It depends on your neighbors, I guess.

Sys-audio?
The whole point of sys-audio is to remove as much as possible hardware from dom0 thus further reducing attack surface.

If you decide to use bluetooth, it’s irrelevant to what app qube you would attach it, but at least make it offline. You saved dom0 anyway.

Maybe something of this would give you an idea