Well, one of the approaches to playing as ERE is to accumulate investment capital and live off of interest (5% per turn). Playing like that, you can survive even just by keeping a few islands in the Mediterranean and launching a counter-attack from there.

Also, a lot depends on the starting traits of Sassanids and ERE's puppet rulers. If they roll reasonable, ERE can be easy even on legendary difficulty. If those rulers roll as ERE haters, unreliable, etc., prepare for hell starting from turn 2...

I personally, played ERE by stabilizing food, happiness, sanitation initially and just holding off my enemies until the econ stabilized. I earned a lot of cash from joining all WRE's wars which eventually raised their opinion of me so high that they volunteered to become my puppets netting me around 12-15K extra income/turn. From that point, the campaign became very easy.