Devastation is automatic. It doesn't give Baktria anything. BTW, I agree that the best plan is to create a buffer zone between you and AS for peace. I tried using Phraapsa, Ekbatana, and Susa under Aedui control as a buffer. It worked ok... but sometimes AS will just never give you peace. If you get it, the 500 mnai a turn as gifts helps - I think this works better as individual tributes, not one mnai per turn agreement.

I would also like to know how the heck you ever saved up that much mnai as Baktra, lol. I don't think I ever broke 10k.