As I generally do not like blitzing, especially those unfortunate Seleucids, I would recommend to stay in peace with them (and make peace with Sauromatae too) as long as possible and try to unite those "rebel" provinces in Caucasus first, thus opening way for Pan-Caucasian reforms, which in turn make your eventually inevitabe confrontation with AS much easier and role-playing-wise more elegant.

Do not forget to read through this guide and try to be true to the spirit of it in your gameplay progression.