SCOTUS simply would grant such broad presidential immunity if a) it was a Democratic President accused of significant wrongdoing, and b) if it was not confident of generational control of the court, which effectively gives it final say on any outcome.
That's key: they create a totally blurry distinction between "official" and "unofficial" acts, to be enforced by them, which allows them to ensure that immunity applies to Republican presidents only. All the jokes about Biden calling on SEAL Team 6 can only reinforce this terrifying point.
If it is willing to read such broad expansive protections for the Presidency, expect it to commit to a maximalist view of unitary executive in a second Trump term, with a broad interpretation of Article II powers (goodbye civil service system, independent agency authority).