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.