I also find myself usually fast forwarding every scene with Donna after a certain point whenever I do a rewatch, both James and her should've been pushed in the background after the central mystery was solved since (IMHO) they're just boring characters.

In my view, this is true only after the first complete watch. Too easy to miss some important details in those episodes if one is aggressively fast forwarding. No one when this first aired even could have fast-forwarded. Better to experience it more like the original generation of us did. (Preferably on an old-fashioned CRT TV... just kidding... mostly. )