I'm also from north east area and ridden around them coastal paths many timescheers, that loks like a good nugget of local knowledge. Will be on CX / gravel bikes so wet mud not ideal.
Amble/Alnmouth
Berwick
Galashiels
I've also had the map out scouting potential snack and lunch stops, which are most important!
Theres plenty of snacks places all along the coast upto newbiging then it starts to get a bit more quieter ( by our standards,,,as our region is quiet anyways ) amble is a nice small seaside town with cafes chippy and pubs but just up the rd is warkworth which is dominated by the castle that was home to harry Hotspur of the percy family he was by all reports an extremely brave knight who went into battle as a very young teen ...I would recommend an overnight stay there and visit the castle and the cave where a former knight of sir percy became a hermit
Alnmouth further up is very pretty and has just won some award in a country magazine
Its then coastal rds upto seahouses and the ferry to the farnes if you wish
Bamburg castle is another must visit along with grace darling house
Bamburgh beach has many unconfirmed rumours that the great 60s film el CID was shot there
If you wanted to slight deviate to the west of belford there is an interesting cave where st cuthbert body was hid by his monks from the vikings who were hot on there tail
The farnes are great but you will have to plan that very accurately due to tides and you may find it take up more time than you plan and may be a better overnight than berwick which was as far north ad I have been on that route
Good luck and hope it goes well