I don't know. I had such a problem trying to get v 2.9 running on buster over a year ago
Interesting, I have noticed you mention this several times. I have run 2.9 on Buster in both VM's and on my plasma table config without any issues.
You might be closer to the development than I am so can time things better. But I had been running my plasma on Mint 17.3 for about 4-5 years as RIP, way before plasmac even started. Eventually, python versions rendered it obsolete and impossible to upgrade. After flirting with MX linux you suggested, it did not support some of my hardware. Mint at the time did not support the correct version of Python 3 so it was not an option. Also the ethernet card on the Odroid H2+ I use in a mill needed kernel 5.9 or higher so Buster was out on it. I went to Buster on my plasma and not long later making a higher version of Python 3 as a prerequisite for 2.9 prevented upgrading at the time.
So a few weeks before Bullseye was officially released, I installed it so there was no conflict between Python 2.7 and 3.x and everything worked. Once Linuxcnc got to Debian Bookworm, I started to experiment with it too to do deb installs instead of compiling from source. In the absence of debs for Bullseye, this was sensible, There have been a few hiccups along the way but generally its all just worked.
My MIll was using ethercat and starting on Bulldeye, I could not get it installed but Grotius made a deb for me and that was quite painfull. After that, I was able to work out how to install ethercat from a Repo maintained by the etherlabmaster guys. Then I tried to get linuxcnc and ethercat installed on a Raspberry Pi 4B. I did actually achieve this and latency was good. but it was a long journey requiring compiling vtk and other bits but started with the official Debian Bullseye.
So it still goes on. The best GUI I've found for my mill is one built with QTPYVCP but it won't install on my older version of Bullseye. I did try QTWoodpecker as it was one of the few that fit on my monitor but the probing is too complicated for a small mill conversion. Shame about that. So this week, I'll be reinstallling Bullseye and ethercat on my H2+ which I'm not looking forward to. Buster is too old to support modern hardware.
