Just to bump this along and show some appreciation for the advice so far here is an update:
Couldn't wait so I purchased a duet 3 6HC, I figured the knowledge gain in the short term would be worth it..... and a panel due... I like screens
Connected it all up following guides on here and it fired up great, all working in standalone config.
Connected up to 24v, all good, got a set of engineer crimps and rewired steppers and end stop switches, all good! (dual y-axis steppers are now in series)
Up to this point all very easy, I'm not rushing and set small daily targets.
The next step, BLTouch, very easy to wire in but had to mess with the home z and home all files, it didn't work right out of the configurator, but got it operating fine as the z-stop in the end.
Next, because I didn't pick up my build platform yet (it's a 5-hour round trip away) I thought i would try to record a z compensation on my acme plywood build surface. This is where I hit some problems. It would getan error about not triggering, so I looked at the BLTouch tip during the process and sure enough, the z-axis would stop before the BLTouch triggered. I tried a few different settings for Z offset but still have not got that going right. I did get a sequence to complete by putting a 4mm sheet of plywood between my temporary bed after the z homing had run, then the compensation sequence runs no problem...
Finally, I have ordered a new hotend and extruder, maybe after I have fixed the nozzle position I can get the BLTouch set with an accurate z-offset.
If anyone has any advice on config for the BLTouch for why I might get this type of problem, it would be great to get pointed in the right direction, this is my first attempt doing anything like this and there is a lot of information to get through so I might have missed something.