None of the traffic controls we rode through were a problem for us, they all detected us quickly (unlike the bridge in Chepstow on The Dean which seemed to have a broken sensor). The one on the climb from Dolgellau to Cross Foxes looked potentially dodgy due to the likely slow speed, so we took the scenic route, which is actually much more pleasant than the main road and I prefer it.
Most drivers were fine, but there were a lot of drivers with trailers and caravans taking a chance on the oncoming vehicle getting out the way so they didn't have to lose momentum, and there was one HGV that passed us then tried to pass the next cyclist up the road and had to pull in and brake sharply to avoid the car coming the other way around the blind corner. They didn't have a lot of stopping distance to avoid rear-ending the cyclist. Entirely predictable, and would potentially have been a lot worse if the oncoming car had appeared a few seconds later. However, the Land Rover driver who swerved over to our side of the road on a straight, relatively wide road aiming towards us was clearly not doing so by accident & was trying to scare us.
We did have to pull off once on the Bronllys to Builth Wells road because a queue had formed - mainly because an overly-timid driver wouldn't overtake on a solid white line, even though we were going slow enough for it to be legal. By the time we found a safe spot to pull in, quite a long queue had formed. The driver at the back understood what we'd done and waved their thanks. Shortly, we passed them all again, and more, in Builth Wells due to the queues.
We gained an advantage around Abergavenny by taking the bypass, which I'd seen Andy & Jasmine do in 2010. This is especially beneficial on a tandem due to the lack of being slowed at junctions in the town centre, and we found ourselves ahead of riders who had dropped us earlier.