There's nothing in the constitution that bans gerrymandering. Actually quite the opposite since it explicitly gives the power to control elections to the state. Unfortunately though as said already, the constitution explicitly bans multi-state pacts without congressional approval.

I'm not a fan of this SCOTUS, but they've clearly shown that they prefer a strict interpretation. They've been swatting down plenty of Trump's executive orders too

so I’m not sure what you mean when you say that they’ve ignored every state election issue this year.

That was not really a State election issue, it was the Constitution and the federal Voting Rights Act conflicting in a way SCOTUS ignored for 30 years, just making an assumption for simplicity (the exact thing SCOTUS exists to determine and shouldn't be ignoring). Since States handle the redistricting maps, a State was involved, but the actual issues at hand were federal, so SCOTUS has jurisdiction.

There is very little in common between determining conflicts with race-based limits and requirements for drawing maps, and reinterpreting part of Article 1 of the Constitution. One of the easiest to read parts

Article I, Section 10, Clause 3:
No State shall, without the Consent of Congress, lay any Duty of Tonnage, keep Troops, or Ships of War in time of Peace, enter into any Agreement or Compact with another State, or with a foreign Power, or engage in War, unless actually invaded, or in such imminent Danger as will not admit of delay.

That is pretty clear cut. And unlike the above situation where we are talking about a newer federal law conflicting the Constitution, there is no conflict here. If Congress consents, then the Compact is approved.