(...) around 195 hours invested on this playthrough (156 hours on 1st playthrough, 162 on 2nd), (...) no compelling enemy, no mystery left in the story, and my party having stagnated, I eventually run out of interest in Act 3. (...) Just can't get into the game now.
So what you're saying is, after 500+ hours of playing this essentially-single-player CRPG, on your third playthrough, you're kind of getting bored. After playing the game for like half a day, every day, for about 3 months, it's getting boring. 500+ hours. Yeap, yeap, I can see how that might happen.
I don't know what the average is, but somehow I don't think the "many other gamers" bit is justified here. It's like a race driver asking for speed limits to be changed because him "and many other drivers" can handle it safely. There aren't that many other drivers on the same level.
You obviously love the game to invest so much time in it. Give it a break. Do something else. Write a guide or three, make some videos, get into modding, whatever. They will probably rebalance the game in a year or two, release an improved "definitive edition" like they did with DOS2. Give them time and replay that one. And by the way, if you haven't played Divinity Original Sin games, try those. They are different than BG3, but similar games.
