taxation is your payment for services rendered by your government. Army, police force, road maintenance, infrastructure. Your taxes pay for all that shit. And if you really want to bitch about welfare, you can consider that the "service" of keeping beggars off the streets and out of your sight/way.
And before you start saying its not a true transaction because you didn't choose what to "buy" well sorry but you did -- via your elected representatives. You can argue that the price is too high or whatever, or that your representatives are choosing to "buy" the wrong services or whatever, but calling it theft is rather disingenuous at best.
I agree with the rest of your post, but this bit is ridiculous. You're essentially saying it's impossible for (democratic) governments to commit theft, because everything a government does is morally validated by the fact that it was democratically elected. Sorry, but just because a lot of people agree with something, doesn't mean it's a moral action. Historical examples abound.