And, thats the win this time. It was much smoother having just done it previously. Seleukids still didn't collapse. But, it was just easier because I'd just done it so I knew what mistakes I'd made the first time. For example I was more aggressive at the start and it paid off.
The big mistake the first time was picking staying Pontic culture though. I'm 99.5% sure that was what doomed the prior run. I wish once I'd picked the achieve box would have "locked". But....eh, whatever.
Always ally Thrace until you've gotten enough land that the Seleukids and Armenia will not have you in "easy victim" category.
Beat the Seleukids at least once before doing the imperial challenge war too. You can milk allies like Armenia and Egypt there. Once you're in the imperial challenge war you want it to be one on one.
For some reason, it seems profitable to become a Black Sea power to do this. Perhaps that is just because the Bosporan Kingdom is overly weak and getting all that Scythian land is worth a minor positive rather than the huge negative it'd have been in the real world.