Penrhos is open again, I met the MOs at a Nant Syddion work party recently.
Dulyn from Mellynllyn and across the dam involves some rock cut steps that you'll have to carry bikes up/down, however the "lower" path can be pretty boggy.
Despite it not being the least remote bothy in Wales Dulyn can be busy and can attract all sorts of characters. I don't think I've had either bothy to myself on a summer weekend.
Also if you're not already a member consider joining the Mountain Bothies Association or maybe turn up to a work party, the Wales area lot are a good bunch and everyone is made welcome whatever their skill level (or lack thereof).