The Strawberry Line between Yatton and Congresbury is useful, I use it regularly on 28 mm tyres, and see plenty of cyclists on road bikes, the surface generally being similar to the section south of Congresbury that you have chosen. Nice moorland section. The B road from Yatton to Congresbury carries a lot of traffic, being a main route south from the M5 juction near Clevedon.
Approaching Pill the paths beside the car pounds are a useful way from Clapton-in-Gordano to the M5 bridge, although you miss Pill, which has some interesting features. (5)
Your route along the Severn levels to the west of the A38 is good. You may wish to consider some local details from the Bristol transit routes (1-3) and the BS9 exit route (4). My detailed local knowledge extends roughly from Axbridge to Slimbridge.
Specific board for this popular undertaking.
On Day 10 you are crossing the Kessock Bridge and turning left down a very steep hill then along the water front and up a slightly less steep hill to cross under the A9. You might consider instead of turning left after the bridge crossing continuing on the flat and level joint user path alongside the A9, until you come to the undercrossing of the A9. Apart from being less energy sapping, this routing takes you via a large layby with a good bakery where you can buy snacks, takeaway or sit in.