Huh, it's nice to see that other people like Total War games as well. I recently (re)started playing Rome 2 Total War. Turns out that the Seleucids have a pretty nice mix of units
That they do, plus plenty of weight to throw around demanding tributes and such. You can declare war on someone, wait a few turns as they realize half the entire world is at war with them (your vassals and clients) and then they'll offer you like 6,000 gold for peace.
That's why you never build a navy with only siege ships. Have your ballista ships do damage to the enemy transports, and as they move closer, move your infantry ships in front of your ballista ships to protect them. When the enemy gets close, order ballista ships to fire at their backline, and order your infantry ships to ram their transports. Then, as their transports engage your infantry ships, order your ballista ships to ram their ships which are now conveniently tied down
But they'll never get tied down because they have 20 transports, which will always outnumber you unless you have a stupidly large navy. The only solution is to build the pursuit triremes which are strong enough to one-hit-kill them if they ram them from the side or back. Ramming from the front twice will also work, but it opens you up to being boarded if you don't do it exactly right, or if there are too many transports next to you.