For whatever reason, I increasingly use ne … que to express “only”, as opposed to perhaps juste or seulement.

Well you’re in luck, this is usually the idiomatic way of saying it.

The three translations you suggest are not idiomatic (at least in France). I don’t think they’re “strictly ungrammatical”, but you shouldn’t use them – a native speaker wouldn’t say that.

Instead I would answer simply :

Tu regardes des films d’horreur ?
Rarement.

As it is implied that it is “only” rarely. Or if you prefer:

Oui, mais rarement.

The “mais” indicating that you’re nuancing your answer, the same way that “only” does.