Fixed it! The default v4l2loopback driver is borkened in 20.04. Grabbed the latest code from the GitHub repo, compiled, installed. Works again now.
Weird choppy video problem that I eventually tracked to indicator-multiload which appears to have a performance problem https://discourse.ubuntu.com/t/gnome-shell-performance-improvements-in-ubuntu-20-04/15972?page=2
Was showing up as a periodic freeze in the video every 1 second. It looked like dropped frames, but seems to be the loop in multiload-indicator doing a sweep of whatever it monitors every second (and updating its display) in a really inefficient way.
That was driving me mad.
Video is smooth again, outputs to Zoom via OBS, so all my stuff is working at the standard I need on 20.04.
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