In the second impeachment, his defense council's argument went, words to the effect "the POTUS sets policy in the national interest, as he sees it. If he sees it in the national interest to overturn the election, and stay in office, that makes it OK."

I don't recall that argument being made - but even if he made it, there's no chance that anyone would actually accept it. POTUS doesn't actually have the power to determine that it's in the national interest to overturn the election and stay in office, and that wouldn't make it OK. He could say that he gets to stay in charge - but we will have just had a national campaign between JD Vance/Marco Rubio vs. Democrat To Be Named Later - eighteen months of primaries and general election campaigning that resulted in an outcome. No one's going to listen to him at that point if he steps in and says, "Never mind, I get to be President instead."

What practical restraint on his actions remains?

It doesn't matter. His actions don't determine whether he remains as President. He can stand at a podium and declare that he will remain President for a third term all he wants. That won't make him President for a third term unless many hundreds and hundreds (or more) people decide to go along with him. And there's no plausible scenario in which they would, because our system isn't set up that way.

It doesn't matter what his defense counsel says, or what he says, or what MAGA Twitter says, or whatever. His term expires on January 20, 2029, and he'll no longer be President after that point. And there's no mechanism that exists that will stop that, regardless of what Trump does or says. He has lots of tools at his disposal to mess around with elections, but none of them can do that.