Donostia doesn't seem to have heavy traffic or any traffic zone restrictions (though I haven;t been there in ~9 years). Stayed in a agriturismo outside of town and drove in when needed. Parking availability wasn't too bad.
Warning about Bilbo is to use a GPS and make sure the maps are updated. Miss one turnoff (or get on the wrong way) on the autopista and you may end up having to go on it for several (as much as 20) Kms before you can turn back. No LTZs there either when I was there but is is a lot harder to navigate around compared to Donostia. Parking can be a bit harder to find here.
My favorite in the area is the French side of the Basque country. Seems just that much more charming and quaint.
Try to fit in a visit to a
sidreria.
https://www.sansebastianturismo.com/...t/cider-houses