The points at which the game transition between acts seem a bit arbitrary (mainly for Act I to Act II), and I don’t see a narrative or mechanical reason to lock us out of previous maps and quests. As far as I remember, previous Baldur’s Gate games didn’t have this kind of points of no return. Why do you think they did it? Do you like it?
That fight is also a lot easier if you focus on using a heavy hitter with a bludgeoning weapon doing damage. I made judicious use of Sanctuary, and made sure any vulnerable characters were out of range. You can use ranged weapons to lure Grym and change its threat assessment.
I cast haste on Karlach, giving her 4 hits every turn. She’d do 60 - 90 damage each turn, then have shadowheart cast sanctuary. All while making sure everyone is out of range so grym can’t do much damage to anyone else out of frustration that he can’t target Karlach. Others still sometimes get knocked down due to his Quake spell, but I was able to make it through that fight on Tactician without anyone falling.
Be careful not to trigger a melee reaction though, since that will cause Grym to recalculate his target and can mess up the plan.
I just love smashing dude with the hammer, what can I say lol