Gerrit/CodeApprove/Reviewable-style reviews are actually designed for exactly the scenario you're describing.
The thing you're missing is that it's helpful to see a diff view of, "What changed since my last review?"
If your review workflow is:
1. Junior engineer makes 15 commits to implement a feature in 300 LOC
2. Junior engineer sends you the PR for review
3. You review and send your notes to the engineer
4. Junior engineer makes 15 more commits and another 100 LOC churn, but PR is 350 LOC total diffs
At (4), the thing you probably want to see are the 100 LOC of diffs since step (3). I haven't tried this on GitHub for awhile, but last I checked, your options are to either view only diff of PR against main branch, view each of the 15 commits individually, or hand edit the URL to get the "what's changed since (3)?" view.
On Gerrit/CodeApprove/Reviewable, they all default to "what changed since I last reviewed?" and you comment on that diff rather than what's changed against the main branch, which is the default on GitHub.