The Southdowns Bunkhouse https://www.southdownsbunkhouse.co.uk/ looks lovely and is well located – UK Cycling Events used to run MTB sportives from Amberley so there are great routes from there. Personally, I’d avoid heading East on the SDW unless you like big climbs as your warm-up is Amberley Mount: one of the toughest climbs on the SDW!
Houghton Forest (Whiteways) is a great place to ride and is only 2k West from Houghton Village… though most of it is closed right now as they are clearing trees. Hopefully it will all be open by March-April. There are plenty of locals who can show you round – you could try the MTB Fanatics FB group, for example.
The YH is at Truleigh Hill… I’m not sure what the riding is like there other than out-and-back on the SDW.