First of all, it’s your vote and it’s up to you to cast it (or not) as you see fit. Anyone saying you have an obligation to use it a certain way is violating fundamental democratic values. But other people likewise have the right to judge your decision.

I would say not voting for one of the main parties would show them that the distinction between them doesn’t matter, and so their leadership does not need to be more or less radical to cater to you. And not voting for even a third party shows that effectively no one can meet your standards, so policymakers can entirely ignore whatever it is you care about.

Protesting with your vote, like all protests, only works when your actions actually motivate the intended recipients to act as you want them to. Otherwise, it’s just performative self-indulgence.