So after installing Little Snitch and basically denying "trustd" any
kind of Internet access, I have been seeing reasonably normal jitter
with Bluetooth enabled.
It's not that Bluetooth stops scanning, but it's not scanning as
aggressively. So after a few minutes, there will be very high jitter
when Bluetooth scans the environment, but it would affect only a single
packet. It's easily reduced its chattiness by 99%.
I don't have any empirical data to support the claim that Little Snitch
has anything to do with it (and I am too lazy to dig further into it),
but the reduction in jitter is massively noticeable since Little Snitch.
Which means I can now run Catalina with Bluetooth enabled and not have
any wi-fi problems.
Just FYI, for the archives :-).
Mark.