I have relevant knowledge of much of the route, mostly reasonably up-to-date.
From Cardiff to Chepstow the plan looks sound enough, but you wouldn't go far wrong at any time of day if you simply followed the A48. In any event, I think I'd probably use the A48 to pass Newport to the south - more space, less built up, fewer traffic lights.
At Chepstow, a cycle track follows the A466 and heads straight onto the bridge. There's no need to go into Bulwark unless you particularly want to. The bridge is great at any time of day or night. The most obvious approach takes you to the northern side, but if you find yourselves on the southern side, don't worry, there's no need to turn back.
The route passing Bristol to the north looks as good as anything you could achieve, and the climb over the Cotswolds at Old Sodbury is the one I'd always do in that area unless I wanted to make my life difficult.
Personally, I'd head through Castle Combe to Chippenham and take the A4 from there. I know it looks a bit unimaginitive, but most of the time the A4 is unusually wide, very scenic, not particularly busy, and you can ride for miles with little opportunity for cyclists and drivers to get stressed about each other. If you do decide to go via Swindon, I strongly suggest that you consider taking the Lambourn Valley from Ashbury to Newbury. This is a scenic marvel, often little short of cycling perfection. The A road from Swindon to Hungerford is pretty, but it can be a bit of a racetrack (for cars, that is).
From Newbury onwards, I'd still say there's no compelling reason not to stay on the A4.
I hope this helps!
