I cycled Dunbar to Berwick in May with fully loaded touring bike. I don’t recall any serious problems around Ayton and Habchester. I went into Eyemouth for lunch and there is a long drag back out of the town on a rather poorly surfaced road. That section could be missed out by using the road from Coldingham.
The bit around Pease Bay is very difficult. Big downhill, with ford at bottom. If you don’t fancy the ford there is a bridge with steep steps either side, which means taking off all luggage. Then there is a very steep section which involved a lot of pushing. Unfortunately, I’m not sure there is an alternative unless you use the A1.