Not really you are spot on! You can only build your power base and declared war for independence. No other way to get bigger (well maybe you cam also colonise a bit I want to).
Anyway for Bactria you have two very early chances to become independent. A possible Maurya Seleucid war is bound to leave the Seleucid severely crippled so you can really strike at that that point (preferably once your overlord has lost a fair amount of troops). This war usually doesn't happen since the Seleucids go for the diplomatic solution. If it does though it is a perfect opportunity to go independent. Even if it is very early and you have not build that much the Mauryan will do much of the job for you.
The other early chance you will probably get is during a Diadochi war. The first war between Antigonids and Seleucids happen (almost) always so you can wait a bit till the Seleucids have moves all their armies to the west and then attack. By the time they have won and reach you, you will be able to occupy much land and so take a very favorite peace and independence. Unlike the Maurya wat the Seleucids are going to be winning this one so make sure you declare on time.
Congratulations now you are independent but be weary since you will also need to down the Mauryas sooner or later.
