So, I created The Seleucid Empire custom nation (using nation creator, and unlimited nation creator points). Extended timelines, starting in year 58 (default earliest year with the mod). In this timeline, they successfully defended against the Roman Empire, dealt with their overextension, and pushed the Romans out of Italy and sacking Rome. Considering it was year 63 BC (according to Wiki, but either way, close enough) they were historically defeated, in my own alternate history of them, they instead won and was given many decades to push back the Romans. On the east side, they lost some land as they focused on the west.
Now, the start DOES have a huge empire under their control. Romans pushed back almost out of Europe (not entirely) where as Parthia isn't much of a threat now. So in a way its easy, however...it does pose its own challenges. I do NOT start with ironman, but instead console in heavy unrest in many provinces (to simulate unhappiness in their conquered lands, which have many Roman patriots) AND I make sure to declare war on both parthia and what is left of Rome, as in my alternate history...both nations have been under siege against The Seleucids.
Of course, in some ways its easy...I start with a ton of land, money, land force and trade. HOWEVER, if you start in year 58 and choose Roman Empire...its same exact thing. The hardest part of starting with roman empire (and my Seleucid empire I created in-game) is not fighting parthia or anyone else, but dealing with mass unrest. Which I simulate with console commands with Seleucids. Then I turn on ironman mode afterwards ;P
I have never seen ANYONE make a custom Seleucid nation...so here is the very first Seleucid empire of year 58 (which actually is the most fitting year if one is to do this, since technically they were defeated some time back)
Also hellenic religion and greek culture
With eastern technology and eastern graphics. I think that is most fitting. I did try Anatolia technology that seemed like it might fit better, but it gets automatically changed to eastern since anatolia is too advanced or something for the time, so eastern technology is closest I guess.
I've played about 10 min trying it out after setting it all up...and wow being at war with roman empire (even if its a lot smaller)+parthia+their vassals and all the unrest...very hard start. Kind of seems harder than the default roman empire in year 58, since I'm in even more wars than just rome vs parthia. But lots of fun