What stops any heavily democratic state file a similar lawsuit against Texas and a few other "swing states" that voted republican ? (I'm putting swing states in quotes because I think the meaning of the words changes depending on who you ask.)

It seems to me that at least 2 points raised in the lawsuit, while they might be technically valid, were also used by various local governments in Texas (non-legislative bodies amending election laws, favoring the party of the local government's party).

The 3rd point could easily be argued the other way around, given the global lack of evidence for election mishandling on all states.

The real problem here is that attorneys don't get punished for bringing frivolous lawsuits, even though they have a significant cost to society. I think that's a massive issue with the US judiciary system. In this case, qualified immunity would probably protect the AG of Texas from being personally liable, which is another issue with the current system IMO.