I'm not sure what un-squashed PRs have to do with half-baked commits. Inexperienced devs tend to make too few commits, not too many. I've never known anybody to make commits that don't compile except by a rare accident. It's one of the few rules of version control that people tend to obey without having to make an effort to change their existing habits.
How do you develop that you don't have broken intermediate working states or thoughts that you can't complete by the end of the day?
And I just don’t know what’s special about “this morning” in your sentence. If theres some important intermediary state you’d like to keep track of, by all means commit it. But to commit just because the day has ended is odd to me.
To give a recent example, I was fixing a cryptographic function and had a bunch of constraints in my head. Dumped that into comments and didn't read them carefully enough the next morning. Immediately wrote a change that subtly broke one of those constraints. Having a restore point was helpful. Similarly, I can also say "this experiment I've been pursuing for a couple days went off-track, let's reset and take advantage of hindsight".
It's not something I use frequently, but it's convenient and free.
But I would amend these WIP commits rather than just leaving them as they happen to occur.
The problem for me is that stashes aren't associated with a branch and I find them very hard to manage. I end up just using stash for very temporary things (like moving code to a different branch). Once something has been stashed for more than 10min I basically forget it exists.