I haven't reread every post in this thread so sorry if this doesn't help out but I would just like to point out some things:
As I understand it:
Units only level about 5ish levels (even heroes). It may not be exactly 5 but that's in the ballpark.
There are two types of "Hero" units, one is called Hero and the other Champion.
A starting Hero unit will likely be around D20 level 11 and go up to about 15 (again, not exact but you get the rough idea)
A starting Champion unit will likely be around D20 level 16 and go up to 20.
You aren't going to have dozens of heroes. So with only a handful of heroes and each one only capable of about 5 levels of advancement I don't think it's going to be too tedious, but that may just be me. It's not like you're leveling them from 1 to 20.
As for power levels I believe the top tier city produced units are around a challenge rating of 11 (again, rough estimate) so a hero will be roughly similar in power to a top tier city produced unit although as heroes they may start with higher stats and also as heroes you can equip them with magic gear and such. The point being they won't be insanely powerful compared to the top tier city produced units (though they will be compared to base units like spearmen)
On the other hand Champions will START out slightly more powerful than a fully leveled up top tier unit or even hero. These guys will be rare but when you get one it will be a huge thing as they are VERY powerful.