Building a culture where discussion around alternate
ways of doing things ('this doesn't block merge, but...')
is accepted and expected (without becoming hostile/nit-picky
or devolving into bikeshedding -- if it's not blocking,
the submitter can always just merge)
Yeah, this is IMO an important thing you want to do in code reviews. Specifically, when it's part of an ongoing collaboration and the feedback can be put to use in subsequent reviews.
We had soooo much blocking nitpicking and bikeshedding at my last job.
The "code artistes" among us would block PRs for these sorts of debatable style issues and other nonessential issues that weren't even remotely blockers IMO. Those discussions were a real sap on productivity and team cohesion. And management was unwilling to give any direction.
(It was a particularly big problem on our team because our test suite was a real pig, and moving code through the build/test servers and out to production could take hours sometimes -- so highly debatable nitpicks could result in literal days of lost time)
When I wanted to give that sort of non-blocking constructive feedback, I always simply did what you mention: I left the feedback, discussed things with the submitter, and approved the PR. Not rocket science. Although, apparently, it was beyond some of our devs' comprehension.