Install a mini-split upstairs, it will keep that zone the correct temperature using it’s own thermometer. It can be downsized a bit as the main HVAC unit will help it out using your existing ducting.
For exact design, I would say have one per bedroom or a multi-zone mini-split that can cover each bedroom.
Many (most?) mini-splits are DIY friendly as well, if you want to save a bunch of money and install it yourself.
Yeah, these also work pretty well, OP. Just make sure you get the ones with two hoses, not just one for exhaust. The single hose ones create a vacuum in your house that sucks in outside air making them significantly less efficient.