Reaching the max level way too early bothers me too, but obviously it isn't a concern for everyone.
There is an easy solution, just like for BG1 and BG2 - installing a mod. I used the lvl-20-multiclass one and got to lvl 14, overpowered, not ideal. For my next playthrough I'll look for a mod that reduces the XP gain to about 75%, so I finish the game at level 12 but reach it much later.
