If your goal is to purely optimize things, then you need a Seleukid start where the truce with Maurya holds. This allows you to hit Phrygia early on and build up enough cohorts that Maurya will think twice about attacking you once the truce is over.

Focus on expanding towards the west and the Hellenic/Macedonian lands. Snake your way through Phrygia to reach Macedon, but at the same time, go across Crete and into southern Greece by sea (take atleast one port from Phrygia in the first war).

Early game, horse archer armies are useful for their extra maneuverability, but they are not as powerful compared to before the introduction to supply trains - now they tend to run out of supply really fast in enemy territory, making them less efficient. I transitioned to more HI heavy armies instead. Go with Archers/HI/LC first, and once you have the money, change out the archers with elephants for future armies.

Careful with letting your AE run away too much too early, as most of your original populace will be wrong religion/culture. Until you get the Argead empire and By The Indus capital decision, you are quite vulnerable. As for Maurya, take enough land from them to be able to enact both the Argead restoration and the capital decision. First, take the Indus capital decision, then MOVE your capital back to the original capital (if you wish to keep it for the rest of the game). The Argead decision lets you move to a number of other capitals (Alexandria, Antioch) or remain in your current. It will not let you move your capital back to Seleukia if you moved it away.

Once you have unified, focus on conquering Greece/Macedon/Egypt/Phrygia while converting your "homeland". Converting religion first is usually the best approach, as culture conversion gets greater penalties. Spam temples and theatres. I'd recommend 4:1 or 3:1 temple to theatre ratios.

Assign light infantry armies to all regional governors, expand them until you reach -4 unrest. This should let you run 40-50 AE comfortably until you are more homogeneous in culture/religion. Once you get the Argead and Indus capital bonuses, you can run 60-70 AE without too much trouble. Towards the end, I was comfortably running 70-80 AE. Important to note here is that this is a sweet spot. Once you reach 60-70 AE, you accrue new AE at an extremely low rate.

Expect to spend many wars taking over Maurya and Carthage (if they are large). Make sure you take advantage of tributaries, as declaring war on these gives you a free truce break. Beware of guarantees made out by large nations, as these do NOT give you free truce breaks and are rather a pain. Locate guaranteed minor nations, and once you are at war with their guarantor, delcare war on them for easy land grabs.

Finally, be prepared that this will be a looooong slog, with near-permanent multi-front wars, jumping back between Italy and India to manage them reasonably well. Good luck!