Actually BG3 stealing system is pretty much the same as in the first 2 games ; it was a bit harder then (you only had one -hard- skill check at the beginning) but in the end it was the same result.
Only change is now people cannot tell if an item is stolen or not, wich is far more logical. Previously even gold had to be in separate stacks iirc, "stolen gold" and "honest gold"...
But yes stealing should be a lot harder and/or impossible for powerful items, and gold should not be accessible to steal until key/chest/kill merchant as you mentioned.
In my last run I stole gold after selling and buying stuff I wanted, the trader was left with literally empty shop, and my party ran away with about 12k gold plus all magical weapons and arrows... :p
