I have to believe that Garland was not expecting to find such highly classified documents. SCI stuff is the is most sensitive of all classified information. To access these documents you must have a need to know and must have SCI clearance which is above Top Secret. These documents are allowed in only the most secure building the goverment has and is protected against any type intrusion. Why would Trump want such documents? This is not the kind of document you mistakenly pack.That's why I - and no credible legal expert - buys the idea that simply declaring them unclassified ex post facto would be a legally valid defense. There's a process for marking documents, and that process exists for a reason.
I personally don't think AG Garland would actually prosecute Trump for mishandling documents - it's just too politically perilous. I think he just wanted the damn documents back in the government's possession. Now, if there's evidence that Trump was trying to sell these to someone...all bets are off. Garland would prosecute.
Garland can also use this example to set up the more important legal prosecution: the prosecution for Trump's incitement of lawlessness on January 6th. He'll probably wait until Georgia prosecutes Trump for fraud.