Thomposn are SMG and STG are assault rifles, they are weapons with different profiles and they should not be compared.
My suggestion was to make create different tech paths for UKF and have bren work similar to bar and Vickers K similar to LMG42
But one could improve the sten instead of messing with the Thompson which is a problematic weapon.
They're both automatic weapons with weird profiles and are both very strong. I don't see why they are not comparable in any way in that sense. There's only one assault rifle in the game and only one SMG that performs like the thompson does so they're a lot closer to each other than they are to any other weapon really.
Bren working like a BAR would be interesting. It'd have to come with a removal of the out of cover penalties to reload, CD, and RA if it's going to be useable at all on sections though.
I guess if stens were massively improved and given to sections with some sort of upgrade that'd be functionally the same. But at that point why is the thompson so "problematic"? IMO it'd just be easier to give them a couple thompsons like the lend lease doctrine upgrade (which, as far as I can tell, isn't overly "problematic") and call it a day instead of making yet another weapon profile for a gun that already has 3.