Contributor Responsibility and Review Cost in the Age of Code Generation

josh:

And I especially wish that you could comment on individual commits and make suggestions there rather than only on the complete diff.

Sorry if this is obvious and I misinterpreted your wish, but GitHub does support per-commit review. You can either view the individual commit via the Commits tab and add comments there, or you can choose commit ranges in the Files changed view using the commit filter list in the top left (you end up in the same view with both ways):

It can be a bit painful to map which comment went where only from the review overview, so it's sometimes best to also read comments per-commit. Frankly, I don't know why one would split changes into multiple PRs that make no sense to merge individually. I think individual, self-contained commits in a single PR are a clean and convenient way to organize changesets.