I believe nuclear secrets are covered by separate rules than the usual classification regime such that the President may not be able to simply declassify them. Whether or not those rules are Constitutional has not been tested.
(It should also be noted that anything the president unilaterally declassifies officially becomes vulnerable to FOIA requests. I am extremely skeptical that Trump chose, while president, to make any of these documents available to FOIA -- although this could be tested by anyone willing to file a FOIA request for them. *grin*)
FOIA requests are applicable to agency records - not personal property. When a President decides to declassify anything, there does not need to be a declaration. The action of putting documents into the public domain defines the action.
Yes, something that endangers National Security is dangerous, but not because of classification, but because of content. That is a separate actionable offense. You don't need to be a President to endanger the Nation.
Even so, if there is a listing of Nuclear sites, as some have proposed, that is still not a problem if it is under lock and key. I note that Hillary did reveal the name of an Iranian scientist who was helping us in her hacked emails, that did get him killed. That is a separate actionable offense that should have put her in prison by now. Nothing was done. Again, if such a nuclear listing was included in Trump's locked storage are, then it is easily accessible by negotiation and would have been remediated without protest. It does not rise to the emergenct level to provoke a raid.
Former FBI assistant director Chris Swecker: "What you're supposed to do when you possess these types of powers that the FBI does and Justice Department does is use the least intrusive investigative technique to get to what you need to get to, [whether] it's information, evidence, what have you. You're supposed to take into consideration the seriousness of the offense, and the impact on the public confidence in the FBI and law enforcement in general. These are codified in the domestic investigative operation guidelines and the attorney general guidelines. What jumps out at me is how that was completely … just shifted aside, just pushed aside. And this dramatic raid takes place over a fairly de minimis offense. Police lights flashing, dawn raid, kitted out ninja warriors outside, 30 agents inside."