So all I have currently is M420 L0 S1 as I mention in my post. Make sure it comes after g28 in case that’s configured to unload autolevelling. Make sure L# matches the slot number your mesh is saved to.
To see if it’s working. Print a skirt around the whole bed. Put an Allen key in the grub screws of the couples between the Z motors and the screws and watch or feel for deflection. The movements will be subtle.
It’s also worth seeing if comparing manual levelling using a feeler gauge or paper sheet and the nozzle tip creates the same mesh as automatic levelling using the probe. plus or minus Z offset of course. If there’s a skew to the manual mesh you need to fix axis alignment.