I'm not against squash commits, yet assuming you're using it to squash an entire PR into a single commit (so that you get a linear history) and given that you a

I'm not against squash commits, yet assuming you're using it to squash an entire PR into a single commit (so that you get a linear history) and given that you also prefer that "the 'why' can go into the overall PR and the 'what' into the individual commits", then you might end up with a 'what' part that can be hard to understood, that's because squashed commits do not point to the exact code change that they originally referred (unless of course the entire PR is very small).