Taxation is theft is perceived as a snarky little quip to most ears. It’s bound to appeal to emotionality in either direction without proper qualification. If you witness that statement draw out an emotional trigger in someone, qualify the statement around consent.

Taxation is, and only can be, qualified as theft, precisely because I do not consent to it.

You may say that you do consent. But within the state apparatus, you have no option to withdraw your consent, at least not without perceived penalty.

You do have agency though. You can opt out of the state apparatus in multiple ways, major and minor.

The premise that taxation is theft is always true then. Whether you’d like to argue that it’s justified theft, well I suppose that’s your right. Just know you’re standing on shaky ground with that premise.