Awww shit. Only now I realized that apparently I have already reached the level cap of Baldur's Gate + TotSC. I just noticed that after winning some ultra hard battles, my party members were still stuck at 161000 XP points. Little googling revealed that is indeed the experience points cap with BG + expansion pack. Oh well, no reason then to try to complete any more sidequests either, but just to rush to the end ASAP.
So much for my idea to level up my party a bit more to handle some ultra-hard encounters. Maybe I just need to turn down the difficulty setting instead, I guess I made a mistake cranking it up to the max earlier. The game didn't feel that hard before, until the werewolf island on the TotSC expansion pack. But at this point, many of the more difficult encounters feel to be mostly about luck. If I get lucky, the powerful enemies either miss my characters enough times so that I am able to kill them instead, or they concentrate on summoned monsters instead of my party members.
Frankly, even though I've now reached the experience cap, my party still feels like a bunch of weaklings. :(