This is on our horizon. We have an Epic to track an overall revamp and adding an option to review file-by-file which will include developing something like what you just described.

Epic: https://gitlab.com/groups/gitlab-org/-/epics/516

Issue for the checking of file seen (including the concept of "unread diffs"): https://gitlab.com/gitlab-org/gitlab/-/issues/24629

If you could please add your thoughts in that issue, we could use that feedback while we develop the feature.

Thanks a bunch for caring and voicing your concerns. It helps us get better.