He can declare a national emergency (how many has he already declared as cover for his republic destroying depredations?), postpone elections, invoke the iInsurrection Act, then throw protestors in jail as Antifa members and insurrectionists.

He can try to do all that. How, practically, would that allow him to remain President?

He doesn't have the power or ability to postpone elections - whether he declares a national emergency or no. State governments run elections, not the federal government. They're going to hold them whether Donald Trump wants them to or not. Those elections are for every office, not just the Presidency - so every elected official at every level of government who is up for election has a vested interest in them going forward. So even in states that might be somewhat inclined to bend towards the President, the folks who run those states don't want to put their own offices in jeopardy when their own elections get cancelled unlawfully.

And if he tried that...well, how would it work to keep him as President? His term expires January 20, 2029 regardless of whether there's an election or not. Is he going to try to enter the primaries and be a candidate in 2028? If not, what happens to the GOP nominee when the entire party coalesces to support them and Trump tries to cancel the election? I mean...play this out. How does it work?

The folly of January 6th is that there really wasn't really any path by which Trump could remain President - even if things had gone to plan, seizing the Capitol and stopping the vote for some indefinite period wouldn't magically make Trump have won the election or remain President. The same would be true of all of the above - it's stuff he can do, but none of it would result in him being President on January 21, 2029.