I have a pretty different view on all of that. I don't think it's fair to rate all of Grymforge on one boss encounter especially when it's pretty easily skipped.
I also disagree that the combat is what makes grymforge unfun. Personally I hate the exploring there with all the traps and weird gimmicks to get around. It might be your cup of tea but personally I like the challenge primarily from combat not from the environment. Maybe I would change my mind if the party movement was easier to manage.
Also in regards to the hammer working only once, a better point to me would be that you feel you have to use it at all. Otherwise, I have no problem with a strange and artificial barrier keeping you from cheesing a boss into oblivion. This sort of thing happens in most games and while BG3 could always be better, it is better in this regard that 90% of games out there.
