I sort of feel, average player should be max level at end of game is a good CRPG rule, which means if do all sidequests and clever solutions that give more xp characters will max out earlier but there's no solution to the second problem without breaking the first part?
I agree with this - and I think the approach I have seen working is that the game tightens the focus towards the end. Even at max level you usually still want to defeat the villain and complete the story - but you are likely to be less interested in various side explorations. At least that's the case for me.
If you have the last 10% to 20% of the game focusing on the main story only, you can make sure the players levels caps in that portion and continue to reward the player with last-chapter gear, and since the story visibly moves towards the end you avoid level cap boredom.
Both BG 1 and BG 2: SoA followed this approach and focus on the main story from Chapter 6 onward.
Unfortunately, I believe that is one the largest issue's with Act 3 in the game. It's the first act taking place in an urban setting, and in an urban setting you want to offer lots of side contents with various NPCs and reward exploration. That clashes with the approach of narrowing the focus to avoid max level burnout. That's why I think the previous games allowing you to explore the respective main cities earlier was the better approach.
