I've seen this presented a lot. That it's not a good idea to tell a guy you are only looking for something serious because that will make him run off. And that is... bad?

No, it is good.

Ofc you don't make it seem like you deaperately want HIM to be the one, but just that he can't waste your time and extract free sex and company without effort and if he's worthy.. commitment.

It's not bad to scare off a time waster. So it's not a bad plan to talk about what you want, and rid yourself of this pickme fear of being "not chill".

Being a coolgirl chill chick is the utmost epitome of cringe. It's like being a doormat but make it progressive.