This summer I reworked the heat trap plumbing on the hot side, extending it first barely above the top of the tank (can't go higher due to low ceiling) then down almost to the cold input:
Hoping this shape, plus the weighted flap of the swing valve, would be enough to stop heat escaping when the pump is not working.
Now that heating season has started I see it didn't stop, the pump and plumbing are still warm/hot, even after a few hours
Only closing the manual valve next to the pump stops the flow, I am afraid that the only solution here might be a mechanical gizmo to open/close the valve whenever the pump is/not powered. I was hoping to avoid that, as I wanted this as simple and passive as possible.