That's really it. I've been finding the same limitations with the default PR code review tools. We currently make do by having a section in our PRs "recommended review order" with a list of files/dirs that should be looked at first, which not only is much smaller Loc than everything else but is the main relevance of the PR.
Within a given project there are inevitably certain files/concepts that are more important, and a better review tool will help emphasize the structure of a large PR to make things easier to review.
I suppose you created a web based front end to GitHub PRs for your team to use? Or is it a local tool? Wondering how yours works.
We use gitlab and there's probably some way to hook up a custom external review tool easily (a bot that posts a link to view the current PR in the tool, for example)
Thanks for the idea!