A PR is a terrible way to review. Every developer should review every commit and look at in context, and do so as part of his daily workflow.
Code review meetings don’t work.
PR code reviews show diffs and it’s up to reviewers to see points from where the changed code is called. This may require loading the branch.
Reviewers should not have to break from their work to review commits. They should review at intervals between their own tasks.
Repeat: every developer in the group should review all commits.