The implication of what he said, in the specific case of a quid pro quo, implies that a POTUS can do anything, to advance his personal interests, as long as his personal ambition aligns with what he sees as the "national interest".

That's not actually the argument he was making. He was more generally asserting that the POTUS could believe that his personal interests in getting re-elected are also in the best interests of the country. Therefore, the mere fact that he was trying to advance his personal interests in getting re-elected does not mean he wasn't also acting in what he believed was the national interest.

This is not an argument that whatever the President believes is in the national interest should be allowed. For example, this specific argument of Dershowitz is not saying that if a President thinks that dissolving Congress is in the national interest and his personal interests, it's okay to dissolve Congress. It's a different point.

Ever since his first term, I have been observing how much the functioning of USian democratic processes, and the rule of law, depend on people of good will, doing the right thing. We have never been so cursed with a megalomaniac who does as he pleases, and says "what ya gonna do about it?"

True, but again not exactly the point. Trump can do what he pleases and folks aren't going to stop him, but that's different from Trump being able to get other people to do what he wants. Trump can stand at the podium and declare that he's running for President in 2028, and no one can stop him from making that speech ("whaddya gonna do about it?"). But he's not going to get on any ballots. The states control the ballots, and unlike the rather stretched 14th Amendment argument, it is utterly unambiguous that Trump has in fact been elected to two prior terms and cannot run for President again. It doesn't matter what Trump says or does.

I think that all this talk just gives him too much power. It's silly to talk about Trump having the ability to cancel the 2028 elections just by declaring them cancelled and that he will remain President - there's no scenario where that could happen like that. Doing so treats him as being more fearsome than he is, which gives him power he does not have on his own. He can do a lot of things to damage the 2028 elections, but he can't create a scenario where he gets to remain President.