Just wanted to say that I completely agree with the OP, and any troll that says, "Lol play better," is just that, a troll. I would go so far as to say that Larian created the game specifically so that people would look around for all the hidden dungeons and interesting side quests. Dropping hints in interviews, and discussing purposeful decisions to make the game bigger, point at a dev team that wanted people to enjoy finding all the hidden areas. Plus, this game is, very loosely but still, based on a tabletop game where exploration is one of the three pillars of the game. Speedrunning to the final boss is not how this game was designed to be played. So, hitting as many hours post level cap as the OP did, is not out of the ordinary. It might not be how everyone plays, but it is very much an expected and viable way to play.

So, not only is being level 12 for hundreds of hours defeatist purely from a game play standard. Worse, though, is that the absolutely insane scale of some of the boss battles available near the end game are the kind of thing that few DMs I know would throw at their players until they were at least level 18 or above. I understand the game is designed to be operated at level 12, so those fights are, both in theory and practice, absolutely winnable. That's not my problem with it. My problem is that the D&D player inside me, which, by the way, is the very reason I purchased the game in EA, one more diversion that helps to satiate my D&D fix away from the table, was screaming that those social interactions and combats were textbook examples of Masters of the World tier play. Fighting a devil in his corner of hell? There's no way level 12 characters should be undertaking something like that. It felt, very much, like the devs wanted to give players the "feel" that their characters were in that Masters of the World tier of play, but didn't want to put in all the additional work it was going to take to allow PCs access to higher level spells and abilities. All of which, once again, points at a hasty push out of what could have been, and in my opinion should have been, infinitely better programed and polished before release.

I, in all sincerity, would have been happier if Larian had just said, "Sorry, we're actually only releasing Acts 1&2 in August, because Act 3 needs some serious work." I'd have been much happier waiting for a non-rushed, polished and complete Act 3, that had the game built for PCs of the levels that they should be for tackling situations that tier of play calls for, than being frustrated with gaining three to four more levels worth of experience and getting absolutely (pun intended) nothing to show for it.