In my opinion the HUD Progress Meter should have a cooldown time, like the overheating has.
I mean it looks a bit strange that it disappears on the end of every level and then appears again on the next one. This GIF demonstrates what I mean.
I still like it to disappear and not cooldown, it will be a bit confusing to the overheat if it is too similar. During delay between waves, the progress bar should disappear because nothing is on the screen.