Is there some good idioms for that? I thought about "Let's get into ...", but I am looking for something that sounds better.

firemageIs Are there some good idioms for that?

Not that I know of. Your idea works, though, so there's no real need to substitute an idiom. "small talk" is possible, but I'd call it an expression rather than an idiom.

Let's talk about [what matters / serious matters / important matters] instead of just indulging in [small talk / chit-chat].

Depending on your circle of friends you may find this a case of "easier said than done". I despair of your suggestion being taken up with any enthusiasm. Emotion: sad

You may have better luck just changing the subject to something you want to talk about and seeing what happens.