That’s a crazy achievement for ERE to attempt! Starting in a corner is much easier.
I’d probably focus on saving Egypt and pushing into Arabia. The problem here is that Arabia is big, and it takes a while to actually moves armies in and out of it… time you don’t have.
First thing first (as in every ERE game) sell all your Christian churches to save money and invest in other things, eventually try to convert back to paganism. Although Paganism has some issues in the late game trading food for money, essentially, is a crucial benefit. Fortunately later updates added the Hephthalite Huns which knocks Persia down a peg or two, although you’ll still need to hit them. Playing a standard ERE game I make Persia “a peaceful desert” if you take my meaning. Probably a good idea to do this again as well. I wouldn’t have to do this if I could somehow break the Sassanid hold on all those client kingdoms, but I have to break that alliance and that usually means killing every last Sassanid king.