Easy retorts:
* Slavery was morally wrong (per almost all moral frameworks), but it was just per the government at the time. You should be opposed to immoral things regardless of how just they are under a government.
* You're still using the wrong definition of consent. Imagine there are two words - ConsentI and ConsentP. You're arguing against the phrase "taxation is done with ConsentP" by saying "well I didn't ConsentI!!!". No shit, we're talking about ConsentP. Get with the program or you just sound ignorant.
* The state does get a special exemption from definitions. That's the point of having the definition of 'just' come from the state. This is basic stuff.
(Specifically, defined in the constitution of the state. The behavior of the agents of the state are not exempt from the definitions.)
[0] steelmanning is the opposite of strawmanning: instead of fighting a fake, weak version of your opponent's argument, you go out of your way to fight the strongest possible version of it - which should be easy if your position is the correct one.