Turns out I made my MTB cassette kill, after switching the crank to 36 and getting a brand new chain from KMC.
It all works fine with no load, it all works fine while pedaling on the bigger cogs but I can't switch to the smallest cogs, they just make the chain skip like stupid, no hope of any adjoostment with the barrel.
So after careful consideration and telling Grok to think harder and make no mistakes, doing some other adjoostments along the way, it looks like a new tiny paypiggery was to be committed.