I don’t think I’ve seen any rear lights that dim – they tend to just switch between static and a number of flash modes.
My favoured light for road use is the Blackburn Mars 4 though there may be something better now.
I’d think anything good on road would be way too bright for anyone behind when you hit the trails. You really want a 10 year old light from a good manufacturer. Or modify one by sticking something opaque over it. Actually, why do you need a rear light off road anyway? If you’re nightriding and following someone you can usually see them lighting up the trail ahead of them.