A huge one is also to assume your opponent will make the best move. Don't just make one move threats hoping your opponent will miss them.
This seems like it should be somewhat more advanced, because players at a low level often will miss one move threats. But it is basically the first principle from which nearly all other good habits directly derive. Why develop all your pieces, not bring your queen out early, etc. Why do all those things when you know your opponent will often just miss something anyhow if you start pecking him with one move threats?
It's because if your opponent does not miss those threats, then you've likely just made your own position even weaker. And as a corollary a one-move threat which improves your position is perfectly fine of course, but in that case you're not hoping your opponent misses it. I mean you probably are, but you expect him to see it. But it doesn't matter - because your position has improved regardless.
As a ~900 Elo player, I feel like point #3 is doing the heavy lifting here—kinda reminds me of that "rest of the owl" meme (not that you're wrong, of course!) :-P
When you're up material? Assuming your opponent is in a tier to leave pieces hanging, we should be talking more than a pawn here, and following the first two rules should hold/build that advantage.
At least, as someone not even 900, closing an endgame when I'm already winning sounds like the easier part. But it's hard to believe that's all it takes to get to 1500, it is not my impression that 1300+ players are losing to big "whoops" moments.
I would agree the "don't blunder" and "punish opponents blunders" are harder than endgame knowledge. However, knowing the basics of endgames is actually important to closing out games. Specifically, knowing KQvK, KRvK, and the "ladder technique" is important.
Without any tactics "take free pieces" probably only gets you to around 1200 (chess.com), but if it includes knight and pawn forks, skewers, pins, and discovered attacks it can get you to 1500. Playing perfectly every game is hard though. I would recommend having more chess knowledge than just those 3 rules before you really try for 1500.
Maybe it's just me, but in an endgame where (for example) it's just kings and a bunch of pawns, even a two pawn advantage is easy for me to lose—I pick the wrong pawn island to support with my king, and I've already lost. Knowing good heuristics in these sorts of situations is something that takes active study IME—"don't blunder pieces" is not sufficient, at least for me.
Learning basic variations of even one opening for white and for black can be extremely useful. There's not much tactics in first moves, but knowing how to set up a solid position will help a lot. Just learn basic stuff, no need to go into the weeds too much. Also the names of the openings are pretty cool
