The core problem with this workflow is that it's rigid around doing things The One True Way. The fact that you have a specific comment delimiter is a good example of that. I feel like this doesn't really have any upsides compared to, say, reviewing code on Github. If you want you can always check out the branch and inspect it locally, but you also have the option of doing it all on the web without stashing your changes if the change is not large. Many online review tools also support go to definition, suggesting code changes inline, etc. All of this means more flexibility, everyone can review the way that works for them, not just The One True Way.
Hmm I don't fully agree with this. By using the PR workflow on GitHub, your One True Way is that people need to create an account on GitHub and use their web interface to make the review.
You always have to enforce some kind of conventions, but I wouldn't say GitHub is the most flexible. With an e-mail workflow, everyone can use the e-mail client they want and review the patches the way they want, without being forced to sign up on a website at all.
You can do reviews in a separate clone, to avoid interrupting any changes you're making (reading the diff only needs a 'git fetch', so won't interfere with anything; but making changes is easiest with a full check-out).
You could use 'git worktree' to manage that; but I'm happy with 'git clone' and 'rm -r'.
