Thanks for the hint, but no, no mismatch and yes the files are being pulled (even looked with tshark if everything comes over properly).

The solution was then much more benign. The stock kernel in the NFSroot where the initrd was produced in was much smaller than the one from the bootable system. This lead into why this was the case and it was missing about 200 non-free drivers which somehow made the kernel stop right before really starting off.

Adding those to the NFSroot and then into the initrd solved the problem sigh

Wasted way too much time there and I still have no idea why 5.10 booted ok the whole time.

or if you are comfortable with command line and some replacement shenanigans, there is always mmv