It just occurred to me that my youngest brother has a WiFi enabled portable HVAC thermostat that he likes.
You can move it around so that your “problem room” will more likely be at the temperature you desire at that moment.

Even with dampers in ducts “balancing” flow to each room is still a challenge because hot winter optimum is not cool summer optimum.

Variable speed fans to replace simple grille covers can help.

My next door neighbor went to great expense to completely separate his two story house so his second story had its own isolated HVAC.

I talked to his installer who remarked to me that adding a 12 inch duct with a variable speed and reversible direction electric duct fan is much less expensive and works nearly as good - but does not give you the benefit of half the house staying cool or warm when a HVAC unit is broken.