I'll be making this journey over the next couple of weeks. Is the costal route via the South Coast still possible with the current timetables?
Assuming you want to do it for free, the following seems to be OK but others may have views. In particular I would welcome others to look through for accuracy. I have travelled on all these routes but not on the same day and not into Sussex since covid (I've used the most recent timetables though).
16: Canterbury (09:30) - Folkestone (10:17)
102: Folkestone (10:53) - Rye (12:41) [You can actually change between Folkestone and Hythe - Sandgate is on the sea front. The earlier 10:33 only goes to Romney, you could spend 20 minutes in Dymchurch if you wanted. This is the most infrequent, the through route is hourly.]
101: Rye (13:08) - Hastings (13:55) [101 is much preferable to the 100]
99: Hastings (14:09) - Eastbourne (15:30) [When I have been on this, there have been issues with buses keeping to time - just weight of traffic; whether that is still the case, no idea)
12X: Eastbourne(15:50) - Brighton (17:15)
So, it is possible. I wouldn't do it, but that is me. The last 12X leaves Eastbourne at 16:55, after that it is the stoppers 12 and 12A.
Disadvantage of going via Ashford is first 'free' bus is 10:20 from Canterbury, the through Ashford - Hastings bus has been cut so you have to change at Tenterden. You would get to Hastings at 14:03 but all of these routes are infrequent (no more than hourly) so there is the opportunity for things to go wrong. There are alternatives - from Tenterden to Rye (312) for instance. From Ashford you can get to Lydd but there is only 10 minutes between getting off the 11 and getting on the 102.