Don't confuse Client states with satrapies. Anything that is integrated into a satrapy is by definition not a client state. In the case of the Ionian cities, they were under the responsibility of the satrap based at Sardis, and are not 'allies' of the Persian Empire. A far better example is Macedon, which was not formally part of a satrapy but was considered an ally of Persia until Phillip II or so.
The likeliest thing to happen in a successful conquest of Greece is that the states that resisted will be the ones integrated into a new satrapy, and the states that allied with them get to retain autonomy. The population of Greece would not be difficult for them to manage, this is an Empire that controls Mesopotamia and Egypt for goodness' sake.
Also, the part about not maintaining garrisons is not so true. All of the important cities of the Empire, especially in the Near East, had semi permanent garrisons. Personally I think that the Ionian cities didn't really matter that much to the Persians, relative to the rest of the Empire.
This is events 150 years or so after the time we're talking about, but in OTL Alexander's conquest, notice how no other state joins in attacking the Achaemenids. Likewise, there is a civil war going on at the time he attacks, but notice there is no evidence of revolts. This is a state that is completely militarily supreme. If the Persians managed to conquer Greece, I have absolutely no doubt they'd manage to get an iron grip on it.