I live about 40mins from Betws Y Coed ( its not spelt Bewts Y Coed 🙄 ) If me and my m8’s have 2-3days of back to back riding we do sum ut like this:-
Panmachno loops 1 & 2 (20miles riding), Then go over to Coed Y Brenin the next day and do both the MBR route and also Tarw route back to back (about 40k of the best single track going IMHO)
would possibly do the Marin if the weathers good on the last day(great views) but if its pants i would not bother. As the marin is ok for a ride out but compared to the others its a bit underwhelming, but does offer great views on a clear day. Or go to Llandegla on the last day, folk seem to slag it off but the new black and red routes are great fun if ridden fast enough and combine the 2trails and you have 24 or so miles of great single track, and the shop and cafe is top notch. and its only 5mins from my house 😆
Dont know where you normaly ride, but be prepaired for rough rocky trails in North Wales.