They are all very negative and critical people, often times judging others or criticizing my parenting styles, family rules and choices
How do your DCs benefit by trying to bond with people like that?
All the clever witticisms in the world will not stop negative criticising bullies in their tracks if they really are as bad as you describe.
Why 3 entire weeks? A weekend would be more than enough.
You haven't mentioned yet how your DH responds to all of this, which is key to this issue. It's one thing if you have a fully supportive DH who is willing to stand up for you and shield you, However, If your DH is spineless and puts up with it, stays silent, or just tells you not to make such a fuss, then you need a conversation about his reaction to this treatment and why he thinks it's justified. If he hasn't got your back and leaves you to fend for yourself then don't go.
I say this because you describe this behaviour as not just coming from one or two people but a whole bunch of them. So even if the clever comments work on one, you have the next lot queuing up for their turn.
Normally I'd say that the bat-back comments are a convenient way of shutting these issues down, but in this case, dealing with a group of them at once, perhaps you could consider abandoning politeness and saying it like it is - "Why do you make such negative critical comments?"
Or that you find their negative critical comments really difficult to handle, and if they can't tone it down, you will give up trying to engage with them. It is a confrontational approach, but is that worse than being slowly picked on?