As for squashing, I believe that everything should be rebased instead to maintain a proper, linear git history. This maintains the ability to bisect and allows a descriptive git history. Temporary branches pass on their history to the main branch.
Even for temporary branches, labeling commits properly makes reviews easier, especially if commits are atomic. Ideally, all PRs are small and easy, but many times more changes than what's easy to glance at are needed. Understanding what and why these changes were made helps to walk through step by step—they're meant to decay since they are quite literally history. Reviewers should review the commit messages themselves and ask for revisions on them when needed. This is hard to enforce and requires thorough buy-in from people to invest in it. There's some proof in the pudding, at least, if you look at the conventional commits usage in the community.
(very, very idealistically—not very practical) To keep commits atomic, each commit should be tested incrementally, at least with static checks and unit-y (read faster) tests. Each commit must past checks prior to PRs getting merged.