I think you have three groups of people (at least) arguing this.
You have some asserting, as you do, that there needs to be some procedure. I don't disagree with that, however, there isn't one
You're already wrong. There is "procedure" and it's clearly spelled out in the law, among other places in the EO. You're arguing that Trump isn't bound by any procedure in the law, and won't even admit to that much.
Declassifying a document is an order, that Trump when POTUS had the unquestioned power to do. A thought is not an order. It's not an order unless and until he communicates that thought to someone else, so that the order can be recognized by someone other than Trump as an order and for someone else to carry out that order.
I've addressed this, you ignored it. But how can an order not put in writing or communicated verbally or in any other way communicated outside Trump's mind even BE....an order? Do we really need a law stipulating that to be an order by POTUS he in fact is required to communicate that order in a way that puts SOMEONE else on notice, an underling, the public, the USG, the court, that Trump did in fact issue an order? How are we, underlings, Biden, the USG more broadly, the military, to know of orders that never left Trump's mind? How long can he keep these orders secret that he only issued in his mind? Until death, and until death he can recall an order he never issued outside his mind and expect the USG to act on this order?
and without that procedure, his power is literally whatever he claims it is (and can at least attest to in court that he did)
It's not what he claims it is. Trump can of course "attest" to anything he wants as a defense, and any trier of fact is obligated to weigh the evidence for that asserted fact by this private citizen with the same presumptions of speaking truthfully under the law as a common murder suspect. Here the documents are marked classified, which is prima facie evidence the documents AND the information in those documents is now and has always been classified. If Trump wants to overcome that presumption, he will need more than "I declassified them in my mind."
Just for example, there are no "procedures" outlined in the constitution for POTUS to exercise his pardon power. That does not mean, and cannot mean, that anyone who he "thought" was pardoned prior to January 20th, 2021 was in fact pardoned. The pardon power has to be exercised in a way to put the current executive branch on notice that Individual 1 was pardoned. Trump cannot assert, and expect Biden to respect this assertion, that Pillow Guy was pardoned in his mind on January 19th, therefore all prosecutions of Pillow Guy by the DOJ must NOW cease. What crime was he pardoned of doing? All crimes? Well, no crimes because Trump didn't pardon Pillow Guy and no amount of remembering he did years later will matter to the current DoJ or court.
You have another group saying he doesn't have the power.
And then the group arguing he does.