Personally, I'd be going with a 'mental illness' defence...and PTSD.
"Your Honour, my client was overwhelmed by all the things that happened to him as President...the BLM riots, Covid pandemic, the mocking and abuse from the media at home and abroad, crises with US borders etc.
He visited North Korea at great risk to his life, to try to save the world from missile attack....keep 'world peace'.
By the time the election came around, and after all his hard work, and the trauma he suffered in the execution of his duties... including being blamed for everything going wrong in the nation/world...and being rejected by the American people, his mind went kind of fuzzy, vague, like it was damaged.
I submit for your consideration that this damaged mind is the reason behind his unusual and irrational behaviour since then.
This 'business' with the classified documents is a very strange thing to do, certainly not something my client would normally do. It's classic child-like behaviour of someone with a mental illness disorder. Taking things, then trying to hide them in plain sight from those looking for them.
You honour, in view of my client's performance during his time as President, the great debt the nation owes him, and in view of his age, I suggest that if convicted his sentence should be confinement to a comfortable home, with rolling green hills surrounding it, where he can sit on a bench outside in the sunshine and pat a dog, be visited by family, and be treated for his damaged mind illness by trained professionals."