* The potential travel hotspots to Bath / Bristol that I mentioned do not fall into a neat line
Maybe we need two lines then. Complement what you propose with Taunton - Bridgwater - Cossington - Glastonbury - Wells - (epic bit of new construction parallel to the Old Frome Road to get onto the
S&D▸ ) - Chilcompton - Midsomer Norton - Radstock - Wellow - Oldfield Park High Level - Bath Green Park
* There isn't a single commuter service from Westbury / Frome / Bruton / Castle Cary into Taunton which may be indicative of the lack of traffic in that direction (but, yes, I don't know what the latent demand is)
With a few reopened stations, a Swindon-Westbury-Taunton local service looks a good idea... Would get rid of messy reversals at Westbury too.
Throwing a stone in the pond I have described ... Exisiting track Bristol Temple Meads, Bedminster, Parson Street, Long Ashton, Flax Bourton, New cut / tunnel Lulsgate, Blagdon, Cheddar Reopened Wookey, Wells, Shepton Mallet New curve to other line to reopen Cole Junction, Wincanton, Templecombe, Blandford, Broadstone, Poole. ... Stands back and expects alternative suggestions via the Radstock area, and around the north of Bournemouth.
I'd hope a regional train like that would not stop at all those Bristol local stations.