I would consider Amberley to Eastbourne and then a train back.
Amberly is pretty much halfway and is easy to get to – with the station right by the SDW.
It’s just over 50 miles to Eastbourne, and you get most of the good scenery… pick a day with decent weather so you can appreciate the views, and avoid days with strong headwinds. You do get most of the big climbs too (which is either good or bad depending on your view of things) – including an immediate climb up Amberly Mount, which I think is just about the longest and steepest on the route.
This is definitely the harder half of the route, so once you’ve knocked it off you will know you are ready!