Total tangent here, but re-reading the ruling has got me wondering where in the Constitution the Originalists found this principle.

I would say the basic separation of powers. If you can drag the president before the courts for any act taken in office, then the president is not the executive; the court system is.

Since you seem a bit more knowledgeable about the subject

Key word is seem, they're talking out their ass. Anyone pretending this isn't a big fucking deal is either an idiot or purposefully lying.

Lower courts decide they can’t determine

That is a nonsensical position. Perhaps a judge determines they are not capable, and recuses themselves or otherwise resigns from the case: the case is reassigned to another judge. But any nitwit can make some sort of decision and support it with some sort of rationale.

The trial court judge cannot "send it up the courts". They render a decision, and one of the litigants - not the judge - petitions the appellate court, arguing that the trial court's rationale was wrong.

And since they set no standards, can determine them on partisan lines.

That is, and always has been, a risk in the judicial system established by our constitution. The checks and balances the legislative and judicial branch have against the court are few and weak.

At best, If SCOTUS engages in such shenanigans, such shenanigans will be engaged against SCOTUS: court packing, etc. Ultimately, though, the only real limit on the court is the willingness of We The People to accept its decisions.

Personally, and this is off on a tangent, I think we are due for a fundamental change to the way we empanel the courts, to reduce the politicization of the court. Instead of fixing the size of the court at 9, I think we should ignore the size of the court entirely, and just appoint one new, life-term justice in the first and third year of each presidential term. Any justice who dies or resigns is not replaced. The courts composition shifts on a slow, but steady pace. It does not stagnate due to justices timing their retirements for when a favorable replacement can be made. Nor does it lurch wildly when a justice gets that timing wrong and dies with the wrong party in the white house.

Further, I would adjust the confirmation process. If the president nominates a candidate who has been previously confirmed to a circuit court, no additional confirmation is required. The president thus has a small pool of qualified candidates he can elevate to the court directly, without needing to involve a hostile Senate.