Suggestions on doing Devon and Cornwall

I can recommend the Cider House B&B in the grounds of Buckland Abbey. http://www.cider-house.co.uk/index.php
This is a beautiful area of the Tavey Valley, you'll pass Dartmoor ponies and it's not crowded. Buckland Abbey was the home of Sir Francis Drake and you'll get complimentary tickets to tour the house (it's a National Trust property). After 28 days last year I think you'll cope just fine!

This may be more than you are looking for but I've done the Road Scholar Cornwall program. It says 10 days but they count the day you leave the US as Day 1 and of course count the departure day as Day 10. You meet up in London for the first night, ride out to Cornwall in a coach stopping in Exeter for lunch and a tour of the cathedral, then are based in Falmouth for 6 nights, return to London for a night at Heathrow as the last night of the trip.

https://www.roadscholar.org/find-an-adventure/22431/Cornwall-England-s-Hidden-Treasure

The long distance ride is in a full-sized coach. The day trips from Falmouth are in a smaller 18-person van so you can get down the skinny lanes and back to cool things.

On the Road Scholar programs, like the Rick Steves tours, I arrive in time to have independent time on my own in the arrival city, so for this one I spent about 10 nights ahead in London at a different hotel then moved over to the Road Scholar tour hotel.

Rabbie's has a good reputation and this tour looks good (though it includes Stonehenge). I can't speak from experience. Cornwall roads can be slow compared to elsewhere and leaving the driving to others is a good way to go if you can. Rail and bus service is another option but building a good itinerary around their schedules would be difficult, not to mention dealing with bags all the time.

Look at the following map - the areas coloured green are the most scenic - but you would need to drive yourself to see most of them.https://www.google.co.uk/maps/@50.6914159,-3.9433723,9.21z/data=!5m1!1e1

These places are very attractive - find them on the map and then Google them. DARTMOUTH, SALCOMBE, POLPERRO, FOWEY, EDEN PROJECT, LOST GARDENS OF HELIGAN, TREBAH & GLENDURGAN GARDENS (near Falmouth), ST. MICHAELS MOUNT, MOUSEHOLE, PORTHCURNO (Minack Theatre - curt into the cliffs), LAND’S END (although commercialised), ST.IVES, PADSTOW, TINTAGEL, CLOVELLY, LYNMOUTH, SELWORTHY, DUNSTER. Avoid Newquay! Truro is fairy attractive and being central, makes a good base for touring Cornwall.

Roads across Dartmoor can be ‘challenging' for foreigners. Exeter is worth a look but can be difficult to drive in and out of due to congestion. Plymouth has the quay from which The Pilgrim Fathers set sail for America but the city is not that interesting/attractive.
Regarding driving or not - zoom down on the map and then select street view for some of the places mentioned. Most have tiny narrow streets and as a result, car parks are found on the edge of towns.

The A39 route along the north coast of Exmoor has many bends & steep hills. Click the following link to get an idea of what this road is like so that you would know what to expect should you drive this route. Click to go forward around the bend and you will come to a hill with a practically sheer drop to the ocean below on one side. At the bottom of the hill is Lynmouth:> https://www.google.co.uk/maps/@51.230878,-3.8013483,3a,75y,227.41h,76.57t/data=!3m6!1e1!3m4!1s0OEibVWncVn725tuT_pBCA!2e0!7i16384!8i8192!5m1!1e1

Thanks everyone for your information.
James, as usual, you provided great stuff for our planning.

We are still not sure that we want to drive again, but it worked the last two times we did Britain.

If you don’t want to drive all the way, consider taking the train as far as Exeter or Plymouth and then hire a car. You have driven in Wales so are well used to driving on the more winding roads. Try and avoid the British school holidays = last week of May and from about 20 July to 1 September.
https://www.visitcornwall.com
https://www.visitsouthdevon.co.uk/places/south-hams
https://www.visitislesofscilly.com