(You'll note that the examples I cite are not states that Texas is suing, and that should give you some indication of the likelihood of success.)
[Side note: that's one of the arguments. The actual brief is kind of a legally incoherent mess, so it kind of hops around from argument to argument a bit; the other main argument, which most of the amici seem to want to focus on, is that PA's Supreme Court improperly adjudicated PA law's propriety under the PA Constitution, and therefore Texas has right to settle that dispute in front of SCOTUS, despite SCOTUS dismissing an appeal alleging exactly that just yesterday.]
It’s a bad argument. The US Congress has delegated some of its power to the Executive Branch (FDA and IRS, among others), just like I’m sure some of these states have. If a state legislature passed laws allowing the governor to change election rules, then the legislature is still choosing the electors, not directly, but by having delegated that power to the executive branch of the state.
I’m not a lawyer but this argument sounds flimsy at best.
2) You should also explain more clearly why you think it could be rational to discount the decisions of state courts which rule on the laws passed by the state legislature on the basis that they aren't the state legislature... That is like saying the Supreme Court shouldn't have a say over how Federal laws are applied. It's their entire job.
And all this is ignoring the idea that the legal system ought to represent the democratic will of the people and the institutional processes they put in place, whether it's via upholding the laws of the legislature or the vote.