While exploring solutions, I use f or ffto mean “follow-up/to-squash” and a to mean logically separate. Sometimes other (additional) short abbreviations to know where to move, squash, and edit the changes to.

I recently discovered git commit --fixup=abcd1234: it will make a new commit with a message of fixup! <message from abcd1234>. (It's the only special thing that flag does: a specially formatted commit message, which you can craft yourself if you remember the spelling of the fixup! marker.)

When you later rebase, git rebase --interactive --autosquash will automatically mark that commit to be a fixup of abcd1234.

magit for emacs has shortcut for creating a fixup commit selecting the previous commit, I'm sure other interfaces do too.

I guess my commit descriptions get better with project lifetime

I've found that too, which I think is because as the project matures, you're more likely to make fixes or contained features, as opposed to regular "change everything" as you explore the design in a young project.