I find the scenery from Southease to Alfriston some of the nicest in the Shoreham to Eastbourne section, but as njee20 says you’ve also got the climbs out of Alfriston and Jevington to deal with, together with the one out of Southease which is very steep at the top.
There’s options to bale between Lewes and Eastbourne though, with a station at Southease to take you in to Lewes (about an hour from Ditchling Beacon), and Berwick station only 10 minutes from Alfriston (give yourself at least 45 from Southease). You could use Polegate from Jevington, but you’re just as close to Eastbourne and may as well finish it!
If you want single track you’ll need to turn off SDW, there’s some of Jenn’s routes around Brighton on the website.