I just hope its not like in AoW III where its more-or-less just a simple race to reach the high tier units.
Didn't that get fixed at some point?
Not really. It was one of the issues that (at least for me) kept AOW3 from overtaking FE:LH as being my favorite fantasy 4x title. It's a pity, as it's a good game overall, with great production values IMO.
I read somewhere (either in an interview or one of the dev diaries) that this is being addressed in Planetfall. However, I don't remember if that's going to be via a hard cap to high-tier units, or something else.