I'm not local to the area, nor have I done a lot of riding there, so you probably know everything that I've done. But I'll chime in anyway while you wait for some decent suggestions from people in the know!
I've ridden Snowdon, up from Llanberis and back the same way, and it was awesome in my opinion. But I did it end of November and it was -12 blizzard at the top! So would be even better now I think. Any many people talk about the Ranger path decent, which sounds like a better (more techy and quieter) decent. The climb from Llanberis is long and techy in places, and unridable in some places for most people. Can't remember how long it took, but I think it was about 3.5 hours or something like that.
Also, not too far from there is Penmachno which is a really great purpose built trail, which doesn't have a whole "trail centre" setup and feels a bit more natural and rugged than most. Think it's a 30km loop.
No await some better suggestions!