As the title says, they currently just keep rolling even if they're either turned off or don't have power.

AFAIK there’s nothing I can do about that without a huge hit to UPS.

Awww : (
But how do belts do that then? I know this is just 2 inserters with the look of a belt like object 'behind?' them, but still.

Belts do it because the belts are actually moving the items. When you connect a circuit wire to a Miniloader, you are connecting the wire to the inserters. This is why you can set conditions and filters on a Miniloader like inserters, not like belts. The belt portion of a Miniloader is a true belt, but not a kind that can take circuit connections, and you wouldn’t be able to configure the belt portion anyway. So the only way to control the belt to stop it when the inserter circuit condition isn’t satisfied is via script, which is terrible on UPS.

How about making the update rate slow? It's just for aesthetics so the tickrate of that script could be slow, like twice a second or less?