7 nights to split between Tuscany and Umbria - but how?

You chose great places in Umbria. Spello may give easier access to highways, but it's only a few kilometers, so not a big difference. However, I would rather come home to Assisi after daytripping, than be two of the thousands of other daytrippers.

I had ten nights to split between Assisi and Siena with friends, we drove to all the places you mentioned, plus Montalcino, Arezzo and Sansepolcro.

we stayed in Spello one year. Small hilltown, very quiet at nite, almost feels like you're back in the middle ages! We've also stayed in Perugia, a bit larger city with more to do/see. Perugia has a carpark just below the town wall where you can leave your car. In Spello we parked at the hotel; note you need a very small car to drive thru the roads in town, it might be better to park above or below the town.

For all your travels in the region, be very aware of the ZTL since all these places are likely to have them and if you aren't careful you can get lots of expensive tickets.

As for time planning, we found that with a car we can visit 2 or 3 towns a day. Head out in the morning, if you like the place stay for lunch and then go on the the 2nd town on your itinerary for the day that's in the vicinity. Back to your base for dinner (we don't like to be out driving late night in rural Italy). If you're not so crazy about the first place then head on out, and then you'll have time for the 3rd.

Urbino is nice to visit. Consider making it a trip to the coast that day and doing a big loop. Pesaro, Fano, etc are smaller towns worth visiting, maybe even Ancona; picking one of them if you go to the coast would be something I recommend. Also allow for serendipity. Some of our favorite towns were ones we saw while driving somewhere else and saw out the window and thought "what's that?" and went to go see. Bring your Michelin Green guide and you'll have an idea of the sights in that town, although we usually just winged it.

Thanks Zoe! Yeah, I agree about Assisi - think that will be our Umbrian base.

Thank John, totally agree about what we refer to as happy accidents. Some of our best travel memories are of places we just happened upon. And I think we will definitely try for the coast, ive never seen the Adriatic and would love to.

Now if I can just determine which region deserves the extra night???

I like Urbino immensely. It is a longish drive from Assisi, however. If you go, consider a stop in Gubbio, another nice town, rather than San Marino. Pienza is charming. I recommend that you take the guided tour of the papal palace. Spello, Montefalco, and Bevagna can easily be combined in a day.
Now for my favorite bugaboo: Civita di Bagnoregio. It’s a tiny hamlet, of no interest, except that it sits on an eroding mesa, and will eventually disappear. Hardly anyone lives there. (I’ve heard the population figure cited as between 8 and 10.), so what you see is truly and strictly for tourists. RS started hyping it a few years ago; and, suddenly, people began to think it rated up with the world class sites of Italy. Of course, a certain amount of commercial development followed, but not all that much. Access is solely by a pedestrian bridge, which is more strenuous and time-consuming than you might imagine. IMO not worth the time and effort, especially when you listed so many other wonderful places that you have in your sights.
If you must, after you pick up your car in Orvieto, drive to Bagnoregio, to the spot above the start of the bridge and view CdB from there.

I would give the extra night to Umbria, roads are twister (is that a word?), driving from town to town can take a little longer.

Thank you so much Rosalyn and Zoe - Umbria it is! Such a relief, I was really working myself up about that 1 night ;) And I think we'll scrap Civita di Bagnoregio for the view only. Such great advice!