I think dividing Rome is kind of a silly and stupid decision. Historically I think that this caused the Western Half of the Empire to collapse. the division of Rome decision is only good if you want the Western Half of the Empire to collapse.. Rome is stronger as one Rome rather than two.

you should use vassals instead of dividing Rome. Use a huge one for iberia, use a smaller for illyria. vassalize either seubia or allemani and give them all your german culture provinces. and do these after you decide to stay hellenism or go christian. for celtic, zalmoxis and nabatean it should be easy for you to convert and control. for caucasus never annex CMG the opm in anatolia called commagene, they're armenian and u can use them for all caucasus, they also have great ideas for tolerance and accepting cultures. for north africa release maurutennia and give all of your african fetishist clay to them. for your starting vassals ditch corduene as soon as possible, you dont need them, annex iberia (georgia) and feed their clay to cmg commagene. the plan should always be that having a circle of vassals and cycling the annexing. annex them, release them ; rinse and repeat. up until the tech 10 where you can change your government type. after that point your empire should be much more stable and only have german tribal vassals on the border of germania, to block the trigger for them to migrate into your lands.

or you can simply de-state everything with the exception of egypt, adopt egyptian as your primary culture and form egypt. ;) keep in the ankh zone up untill the medieval ages, then form Rome again :D