Installation Nightmare! All out of ideas! :-(

Tommy has the link and Robert has good advice. 
Python 2.7 is deprecated on Debian from Version 11  (bullseye) on. Its not possible to then install V 2.8 on Bullseye or Bookworm.
Linuxcnc adopted python 3 over 12 months ago. Its really up to the GUI developers to support Python 3. There has been plenty of time.

I have installed qtpyvcp and probe basic on Bullseye and Bookworm with Python 3 using the  instructions on the QTPYVCP site several times..
I might add that I could not understand what all the fuss about Probe Basic was. It did not excite me,

For me, the whole attraction of using Bullseye or later versions of Debian is to eliminate Python 2.7, better hardware support and because the features I want to use are in master branch,

if OP goes into the BIOS/UEFI settings, is secure boot enabled ?

Hi Robert, I do actually have my machine up and running perfectly with a Raspberry Pi4, and I have tried all the default interfaces. I am putting a probe on the machine and Probe Basic seems to have that side of things covered! . It is probably hard to believe from my ignorant questions, but I am actually very experienced with linux, and write industrial Python code for a living!

my apologies, the posts seemed that you were struggling with the basics. I've not put linuxcnc on an rpi4 yet, have considered it for my lathe, but seemed too many issue posts on the forum.

with your experience you should not have an issue using the linuxcnc iso. I myself don't use the iso's any more when I have started from scratch and just get the Debian non free firmware iso (bullseye), use git for a local master build and build the debs and install. I have after dabbling with other guis gone to use axis as it just works and it's simple

No offence taken Robert, I find that starting any engineering discussion with 'Explain it to me as if I know nothing' is often a very good place to start! .
Now that I have linux installed (on both devices) I should probably start a new thread in the Probe Basic section?

I did briefly try the Pi 4B with the 64 but bullseye version of Pi OS. I found a preempt_patch which installed ok, QT5 installed OK. But VTK did not run. I also could not install ethercat which I think was related to the RPi kernel not being in step with a normal Bullseye version 5.10.

So being unable to run QTPlasmac or Ethercat, I gave up as it was of no use to me.

But aside from all that, the latency was remarkably good, similar to a J1900 or I3. If you were just using gmocappy or axis, it is worth considering..

I'm a BIG fan of the little Pi's, we use them for all kinds of amazing stuff like multi million dollar data systems in mining where a Raspberry Pi is the ONLY interface between the production PLCs and the production reporting! . I also use one for switching guitar patches on the Fractal Axe processor in my band!
BTW, I just posted some info on my build in the 'Show your stuff' section.
Thanks again for the help.

Sorry, one more thing, is there some secret to getting wifi working in this Debianedu12? When I open a terminal and it doesn't understand the command 'ifconfig' my heart sinks!

I think ifconfig needs to be installed: sudo apt install net-tools
technically its depreciated. 
ip a gives a lot of info try it first.
 

Everything is so different in the distribution, I feel like I'm starting again!
I don't see any wifi interface listed using ip -a. I know it has one as it was working when Windows was still installed!