Reviewing large PR's is hard. Breaking apart large PR's that are all related changes into smaller PR's is also hard.

If I submit a big one, I usually leave notes in the description explaining where the "core" changes are and what they are trying to accomplish. The goal being to give the reviewers a good starting point.

I also like to unit test the shit out of my code which helps a lot. The main issue there is getting management to embrace unit tests. Unit tests often double the effort up front but save tons of time in the long run. We're going to spend the time one way or the other. Better to do it up front when it's "cheaper" because charging it to the tech debt credit card racks up lots of expensive interest.