As long as said documents weren't statutorily protected, the argument will be that the President has very broad powers of declassification.
Listen, the system works one of two ways:
1. The President has ultimate authority on classification matters, but their decisions still need to be enacted. The security apparatus still has to be informed of the President's decisions, and it will take some time to implement those decisions. The process of classification is important. Classified markings are important.
2. The president can classify and declassify with a thought, and does not even need to tell anyone. The President's mere thoughts carry full legal weight, and therefore control consequences for ANYONE handling classified documents.
#2 is idiotic for countless reasons. As the most obvious, the President could unilaterally declare that all of YOUR personal documents are classified and have been classified for years. If it only takes a thought, no one can contradict that. Legally or otherwise. You're now on the hook for illegally possessing classified documents. You weren't informed and they were never properly marked? That's no excuse, tough shit.
Anyway, Donald Trump can argue #2 is the way things actually work. My point is that EVEN IF HE'S RIGHT, the documents he took WERE "statutorily protected". President Biden's DOJ had a right to demand and seize them because the President's whims are law. Private citizen Trump had no right to deny them.
The reason we are here is not because he had any information at all, it's because he thumbed his nose at them and was a dick about it. And most importantly lining up a run. Hillary set that precedent of nothing to worry about and it was cemented by Biden and Pence. If having classified documents that you shouldn't have were truly a problem, there we be a lot of ex-presidents worried. Not to mention probably everyone at the federal level that has likely stashed something intentionally or not.
You are mostly correct about the bolded. If Trump had just given the documents back when requested, we probably wouldn't have even heard about the incident.
That is how the system is SUPPOSED to work. Leniency is generally offered for reporting mistakes and cooperating. I'm sure the leniency is greater for bigwigs near the top, but this is true even for peons. Think about it: there are tens of thousands of people handling classified documents every day in this country. They may be well vetted and well trained, but mistakes still get made. If every mistake landed you in court, few would take those jobs and those that did would not destroy their own lives by reporting every mistake. National Security would suffer under those circumstances, so the rules are softened in practice to encourage self-reporting and cooperation.