>- are easier to debug (and possible to bisect; it's not possible to bisect a squashed commit).

In practice, if bisecting indicates a commit caused the problem you can narrow it down further by checking out that branch and investigating further within that branch (which ideally isn't too large). Also, if you have a highly structured PR then you may need to be careful to ensure that each individual commit passes CI.

At that point you might as well ship each commit via a separate PR. (While in development you can temporarily set the branch for Part 1 as the base for Part 2.)