6 nights in Devon - no car - hiking/biking

Totnes is lovely and a big plus point is that it on the railway. You can also take river cruises down to Dartmouth.
If you do stay in Totnes, suggest you take bus 164 down to Salcombe:> see https://bustimes.org/services/164-totnes-kingsbridge

Chagford is nice but the transport options will be more limited.

https://www.dartmouthrailriver.co.uk/tickets/round-robin

You might find this article of interest

https://www.theguardian.com/travel/2021/aug/11/bikes-boats-betjeman-car-free-break-wadebridge-cornwall

If you are using buses in Devon often a Devon Bus day ticket for £11 is best, as it is valid on all operators. Even with the current £3 fare cap, that pays after the 4th bus in the day.
Regarding Salcombe the estuary is a great place for boating.
There is a ferry from Kingsbridge to Salcombe. The sailings are very tidal and are only announced a month at a time. At the start of the month that can be a bit nerve wracking. Take careful note of the departure point in Kingsbridge, as there are two.
Then in Salcombe there are two cross estuary ferries.
You can take the Totnes to Dartmouth ferry one way and the other on the Stagecoach 92 bus.
Or even better come back coastwise on the 93 to Kingsbridge, it then continues as a 3 on the rural run to Plymouth.
If heading up Chagford way one thing to know is that Exeter has 3 stations.
The most convenient is Exeter St Thomas. Change from a main line train at Exeter St David's to a half hourly Paignton train.
Buses to Chagford are run by Stagecoach Their bus 173 refers- 4x a day.
But I think Moretonhampstead is the better place to stay- also on the 173 with extra trips on the long rural route 178 via Chagford to Newton Abbot run by Country Bus. But Moretonhampstead also has a far more frequent service from Exeter St Thomas on the 359 run by Country Bus.
To reach Widecombe use the Saturday only summer service 271 by Country Bus from Newton Abbot railway station (starts tomorrow, until 1 November). So do that while based in Totnes.
From Widecombe it is about a 1 mile walk to the farm.

We did a three day trip to Devon without a car back in May. For us, we found it easiest to stay in Exeter which has some of the best transport links (and is on the train directly from London). We visited Dartmoor by taking a bus to Chagford and then walking to Okehampton where we got the train back to Exeter. (Okehampton is the only town in Dartmoor with a railway station- it is on a small branch line from Exeter.) We also got the train down to Dawlish Warren and walked back along the coast to Exeter. Both were very enjoyable walks, and we also enjoyed visiting the town of Exeter itself (especially the cathedral). Last year we also spent a weekend in Paignton, which is also on the railway line. We loved visiting Agatha Christie's house (Greenway) tho we needed to book a taxi to get there.

One challenge we found in May was a lack of bus service on Sundays between Exeter and any of the towns in Dartmoor. They may run more in the summer. My personal preference would be to be based near a railway station if you plan to travel around the area (places with railway stations also tend to have more bus options).

don't forget that the train ride from Exeter to Newton Abbott has excellent coastal views in quite a short ride. And the trains run on Sundays.

You will love Devon! I had such a wonderful time here, and honestly, I enjoyed it more than Cornwall. I especially liked Dartmoor National Park, although I did have a car, which made it a bit easier.

If by chance you get to the place you mentioned (Higher Venton Farmhouse), definitely stop by St. Pancras Church located in the village of Widecombe-in-the-Moor there. I first heard about this church after reading an article about Bill Bryson, who named it as one of his favorite churches. It's a 13th century church and has a lot of history. It also has an immense steeple, which you can see for miles around. They also have Market Day on Saturday if you're around, with some very interesting things for sale. :-)

I enjoyed the town of Chagford, but it is small so I'm not sure how it would work for public transport. If you do get there, I recommend stopping at the Three Crowns Pub for a meal. The food I had was excellent and their sticky toffee pudding was the best ever. :-) Totnes was interesting although I was there on a Sunday when a lot of places were closed, so I didn't see a lot of it.

My favorite place was Coleton Fishacre, which was the vacation home of the D’Oyly Carte family, of The D’Oyly Carte Opera company fame, who produced most of the Gilbert and Sullivan operettas back around late 1800’s. The family also owned the Savoy Hotel and Claridges in London, which were two of the most renowned and prestigious hotels there (and still are). It was one of the most lovely houses I've been in. It's not historic—the family built it in the early 1900's but it's absolutely beautiful (as are the gardens), especially if you like Art Deco. It has a gorgeous Arts & Crafts exterior, and the interior rooms are furnished in the Art Deco fashion of the 1930’s. It was really lovely and I'm so glad I went. I highly recommend a visit if you enjoy interesting and beautiful architecture and decor.

Without a car, I would think that Exeter would be a good option. I toyed with the idea of staying there but ultimately went with Plymouth. But again, I had a car so that made a difference.

This is all super helpful - thank you. I will report back as our plans develop. Thank you.