and you absolutely CAN defend against all other powers while pushing your conquests forward!
it takes around 8-10 mediocre units to hold off a pontic invasion, and you should be able to hold off celtic, greek, and all other invasions into thrace with even fewer units. moreover, your best units will be produced in thrace, so you wont have to wait a long time for superior reinforcements.
(also, remember that you can REBASE a bunch of units. and that combined with transports and Royal Roads means that you can have an effective defense force reach almost anywhere in your empire, other than india, almost immediately)
finally, im completely against locked alliances as then you cant conquer that power later! ill admit, i've never conquered all the way into gaul, but i've created an empire consisting of Italy, arabia, all of greece, ponus, and ALL of india, in addition to alex's historical conquests. And this was all took place before 323 BCE. you can absolutely go nuts and conquer everyone in this scenario (if you have the patience), especially once you get a large powerful cavalry force.