Quoting this before having finished the thread so apologies of any thematic repeats etc.
The few times I've been to Europe musically or theatrically Ie in a professional capacity where a visa will now be required won't be situations where a visa will be granted.
Fringe, small scale, club level, start up, etc etc venues, artists, bands & performers won't get visa's granted.
The American performers I've played with here haven't come with a visa they've turbed up on "holiday" and borrowed everything they need.
Payment is cash and just goes into the "holiday" kitty.
This will be the same situation now for Eu artists coming here and be extension us going over to mainland European.
Exactly as you say PeteB club level gigs in mainland Europe were financially viable, perhaps even moreso than in the UK. This simply will not be the case from 2021 onwards