In the southern trail centres (afan, cwmcarn) you’d be better taking the bike inside with you. Both are very close to large cities, and are in deprived areas. (You wouldn’t leave your bike in the back of the car in Manchester, London or probably even Milton Keynes)
Cwm Carn is worth doing, maybe try to hook up with some locals though as there is a lot of good riding close to cwm carn. (if you fancy a night ride you can join the monday or thurs Castle Bike rides from Caerphilly)
Afan, both skyline and W2 are currently open, I’d do W2.
Brechfa – the black was closed recently, not sure about the current status, but the red is also fun.
Then head north to Nant Yr Arian, then up to Mach for the Climach trail, finish at Ceod Y Brenin, which AFAIK is all open.