Maaser is tithing, not taxes. Taxation is state-imposed and is not a mitzvah. Maaser is a mitzvah, and is given from HaShem. Two totally different things.
Not in a Theocracy; Or are we ignoring “Som Tasim Alecha Melech”?
Beis Din (i.e. the legal system) would force it if required, and that makes it a tax.
Also, I disagree that Taxation is not a Kiyum Mitzva (in some form, what I am not sure), see BB 9a.