UBL / SKR1.4Turbo / TMC2208 NO leveling correction

UBL never worked for me well, or at all.
This is the case again, so I have tried ABL. It works well but not 100% and there is no possibility for manual adjustment of the grid, as far as I see. (If I am wrong let me know)
At least it does the correction, however still not great at one side of the bed. 90% of the bed is OK but not the right side and I cannot do so much about it.
I am using BLTouch on all my printers. (well, it is not true, one is manual because I was a bit pissed off and take the sensor off )

So, I tried again UBL on my modified CR10 MAX because I want a perfect level. Some of our models are huge, but not high, so good levelling is very important. Also, I have no time to check the printer after it starts printing. With 6-7 printers it is not livable I want good printers and I am working hard to reach this point. So far, in the last 2 years, I never needed help, I could figure out almost everything by myself. - Plenty of help on the net but not for everything. Most of the material is for beginners, and it is great, but not helpful for me anymore.

However, now, this "UBL does nothing" syndrome, well, it testing my patience . Z doesn't move a bit at printing.
Of course, I have saved the mesh, L0, activated, I know Gcode and using it in my start gcode (G29 L0 / G29 J2) in slicer or I write it in manually whatever.
So I cannot see a mistake or problem here. In pronterface I re-check the mesh, and everything looks OK.
However, it is NOT.

And to be honest, I have no idea what can be wrong if it is not Marlin or the Board itself. I cannot find any thread with the correct solution for this.
Thank you for your help.

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The “impossible” does not exist, maximum such things that a human cannot imagine due to its momentary barriers!

I'm not a fan of the BLTouch. Just not reliable enough to keep the head from crashing into the bed when probing. I ended up with a Duet Smart Effector on my delta printer. No more head crashes during probing!

I don't use the G29 J command. Never found an explanation of how it works or why/when it would be useful. Just tried it on my delta. Sure screwed up the matrix!

I'm absolutely convinced that UBL is compensating. I've mounted a dial indicator on my head and watched it when doing X/Y moves. With UBL engaged the fluctuations were in the 0.05mm range (if my poor memory is correct). With UBL disengaged it was at least 10x worse. I can't see the Z movement when UBL is engaged but I can feel it on the lead screws on my cartesian printer.

I'm also convinced that enabling MULTIPLE_PROBING and EXTRA_PROBING really helps. At least it helps the M48 results.

I agree that a UBL grid, that hasn't been fine tuned, isn't usually adequate for a large base area object. If I have a challenging build coming up, I'll spend an hour or two using G26 and G29 P4 to fine tune the mesh. What's disappointing is that 1-4 weeks later I'll have to start using babystepping to be sure the first layer is good.

Why is continuing to get a good first layer in the long term such a pain? I think it is a combination of uncertainty during probing, belt stretch, wheel wear and mechanical warping of the frame (due to thermal cycles & temperature differences).

I don't see how people with printer farms don't go crazy with all the maintenance needed!

I just realized that it took me 3 hours to say "I don't know."