Long story short I think the solution I posted here is fine and it’s only additional stuffs that I was testing but didn’t post yet in the solution that might interfere with the update process.
There is indeed an additional optimization that I never posted in this solution, only started to discuss it in the FURTHER IMPROVEMENTS section: additional symmlinks on singular files. I only tried that on my PC to see what would happen because I had a feeling this could be problematic with updates. That was just before the 1.19.9.0 update.
1.20.6.0 installed in 3 or four passes only, much better than the 60 or more passes, starting and restarting the game for 1.19.9.0. After that I did also need more than one pass to successfully install certain updates on the world updates 1 to 5 packages. But nothing too bad.
Note that I didn’t touch anything since before the 1.19.9.0 update, and I didn’t have a problem with the update before where I implemented the solution but I didn’t yet implement this additional test on single files symlinks. So I conclude the symmlinks on folders are fine, which is what is in the solution, but perhaps the symmlinks on singular files, which are not in the solution posted here, are maybe screwing something up during the update (if so, that would be when unpacking a downloaded package or right after). Anyways this test of adding those symmlinks on singular files did not decrease the loading time very much. So I give up on this additional test on single files symmlinks: not worth the trouble.
Just the folders symmlinks seem enough and without causing problems with updates. Which is what’s in the solution, no more, no less.
On a side note, I have started installing many mods on Cyberpunk 2077 using the application vortex from nexusmods.com, and i noticed it also relies on symmlinks. So I am wondering if perhaps it’s possible to use vortex in order to simplify the installation of this solution (probably not, but that’s just an idea I wanted to throw in the air).
EDIT: This is now explained in the opening post under FURTHER IMPROVEMENTS in paragraph:
ALREADY TRIED AND GIVEN UP