This was a pretty nice route from Bristol to London avoiding the worst bits of busy main road: https://connect.garmin.com/modern/activity/575820070
If you want nicer and dont mind the extra miles/hills, then from Marlborough you could go down through Burbage, Pewsey, Devizes, Bradford on Avon to Bath and then from there follow the sustrans railway route. The Eastern section of this is pretty good either way.
Take the Kennet & Avon canal path, surface is reasonable, certainly from Devizes westwards to Bradford and on to Bath. Cracking scenery. Zero traffic bar the other cyclists and peds. But if you are not in a hurry...
I'm local and I don't recommend section between Bradford and Bath on a weekend or nice days during school hols.