That a literal case of stealing state secrets and refusing to return them could possibly be shielded by this ruling demonstrates just how insane it is.
The layers of protection provided in the Supreme Court ruling makes it very easy to justify either immunity or simply excluding all evidence of a non-official crime. It's much broader than just "official vs. non-official". The LegalEagle YouTube channel has a good run through of all the ways in which immunity or effective immunity could be achieved and it's bad. "Top 5 worst Supreme Court rulings ever."
It may not matter at all. Imagine it's deemed unofficial and he wins the election, he will almost definitely pardon himself. Pardon powers were never given reasonable limits.
People have undervalued that the real check on a president's powers is in the hands of the voters. The next strongest checks seem to be the lifespan of the executive and the two term limit.
Because the ruling quite literally says that anything the president does as the president is presumed to have immunity unless the prosecution can argue that applying criminal law couldn't possibly impact on the core work of the office of the president, and that their motivation for doing what they do can't be considered when making that determination.
"As the president" Trump asked the justice department to falsely claim it had discovered election fraud as part of a plot to steal the election.
The supreme Court ruled that this is protected because if you ignore his motivation, punishing the president for consulting with the justice department about election fraud would clearly impede the core functions of the office.
Without considering motivation, would punishing the president for transporting documents he has legal access to to a place he's allowed to take them impact impact the function of the office?