Stiniog is great, and whilst it’s 4 hours from BPW its ‘almost’ due north so if you went there on your way back to Scotland it might only add an hour to your trip, some epic scenery there too.
I don’t want to take anything away from AS, but it’s like BPW on a smaller scale, less trails and shorter, but they do have some FLIPPING MASSIVE rocks to ride, fun times, but for me it’s too similar to BPW for OPs needs.
Day 1 I’d probably suggest Cwmcarn as its close to BPW and if you’ve been travelling down you might not want to be faffing about on unfamiliar group with the GPS. Cafall is great, Twrc is better but mostly closed and diverted at the moment.
Day 3 try the GAP if you’ve got the legs left for a scenic tour over hill and dale, Afan if you want easy fun, or FOD is the same sort of travel in the other direction.