I don't know how but we need to get developers out of thinking of PRs as the smallest possible change unit. This is literally what commits are for, you do a chunk of work, and you have a commit which describes that chunk of work. If you've got cleanly isolated commits then when you come to reviewing the PR (or changeset as I'd much rather see them called) and someone questions the wisdom of including that particular change you can either modify the commit to satisfy the questions, or just pull it out of the changeset into it's own for later review without blocking merge of the wider feature.

Coincidentally jj makes this process much easier than it would be with git, it will very happily let you shift commits around between different branches, edit commits in place and cleanly rebase those edits onto subsequent commits, or split a messy commit into two commits that makes sense.