Medical tests by the government's Dr,(not their own) releasing taxes, and stricter absolute restrictions on stocks and business dealings while in office should be a requirement to hold office. First one you break takes you out of office. Mental health related to dementia exams should happen annually for younger Presidents, and every 6 months for presidents past the age of 65. Tighter restrictions & time limits on returning classified materials after you leave office should exist as well. They should be on penalty of law, with an immediate jail sentence if not met to the letter.
You do this, and enforce it strictly with every president, then we won't have documents out there to be bandied about. We will have a better idea about the mental and physical health of our President. And we will have a much surer idea of the President's susceptibility to graft & corruption. We have left things open to interpretation on many of these issues, but now it has come to bite us on the tail. If we have no sense of the honor system in our Presidential office holder's, we need to make it a requirement.
If we take out of the Presidents hands, and go as far as putting a President out of office or in jail once or twice, they will hold to these regulations a little more resolutely. All of this should be put in a Presidential Regulations Amendment. I guarantee it would have a better chance of passing than just about any other Constitutional Amendment.
Granted if there are signs of senility and/or dementia there will have to be a wide latitude of how to deal with it, after diagnosed. Early on, those problems don't necessarily hinder the President enough to have to step down. There should be some testing guidelines set, that the President has to meet, before he can be taken out of office. Those guidelines should be lenient. When he doesn't meet those guidelines late in the process of evaluation, it should be put to a large panel of doctors, (maybe 10 doctors well recognized in that field) and require a super majority of 7 out of 10 doctors to say that he must step down.
Side Note:
We should do something similar for the Supreme Court.