Your mistake is assuming that taking property through the threat of violence is theft. That’s incorrect. Theft is an illegal taking of property. Taxes are a legal taking of property. But collection of the wealth tax, like all taxes, is premised on the threat of violence. It’s legal violence towards a legal end, and therefore justified, but it’s still violence.
This comes across as wealth apologism. If a wealth tax is theft through a monopoly on violence, then all tax is theft through a monopoly on violence.
Singularly applying this concept as a the negative of a wealth tax, while ignoring the rest of taxes, is biased towards the wealthy.