You might try adding this branch protection rule to require conversation resolution: https://docs.github.com/en/repositories/configuring-branches...

This feels like a suboptimal solution to me because I personally like to keep comments in "unresolved" state so that they remain visible and other folks can weigh in on them if they want, but in a way that doesn't block the PR. Basically I wish that GitHub would either separate the "collapsed" and "resolved" concepts, or add this "approve without merging" button.