Guide to Egypt:
- Raise the merc army in Alexandria, as well as your own levies, and get them on the Antigonid border
- Wait until Macedon and Antigonids start fighting
- Attack the Antigonids with Legacy of Alexander wargoal
- Use the mercs and main region levies to drive back their armies, and the other levies to do the carpet sieging behind you
- Make sure you carve a line up to the north so the Seleukids can't carve off more than a few border territories (they always join in on the Diadochi Wars)
- If you're still alive by the time you clear out the Antigonids, eat Thrace and Macedon, too
You now control Egypt, the Levant, the Antigonid portion of Asia Minor, the lower portion of Thrace, and Macedon. You are the largest empire on the map, and you're about a decade into the game. You will need to spend a little while consolidating that mess (I recommend spending a lot of innovations on AE reduction), but you can pursue missions at your leisure.