The essence of the case is as I wrote a month ago, the Constitution gives State Legislatures the exclusive right to determine how Electoral College electors are selected, including voting rules.
It does not give any role to the State Executive (Governor and Cabinet) nor the State Judiciary (Courts).

In all of the Defendant States, the Executive and/or Judiciary extended or changes the rules that the State Legislature had made in ways that assisted the Biden campaign (eg allowing mail-in votes without reason, sending out mail-in ballots to everyone etc).

If this is all the case, then it seems like it should not be a difficult case to argue.

However, the bigger question is that even if SCOTUS agrees with Texas, so what? What is the remedy? There is no way to order a new election, and according to the Constitution, a President needs to be in place by a certain date, while adhering to that.

Could SCOTUS reverse the decisions made by the different states? Again, even if they did that, how practical could that be? How do they know which ones were valid or not?

My point is the remedy is as big a mess as the present course of action.