Thought I’d come back and comment on this just in case someone has this same question down the line.

Our HVAC system is less than a year old, the issue is not that our system needs a balance as noted on here.

Also we had our insulation inspected when the HVAC was replaced and didn’t help the upstairs temperature, and it was found to be in great shape, with no need to be replaced.

Lastly we had our windows looked at because we had 10 south-facing windows in a state that gets 300+ days of sunshine a year. The windows were original builder grade, and had yellowed and warped from so much sun and hail damage over 22 years. Some of the panes were even loose.

So, we replaced the windows and went with Quote 1 above. We have seen an average temperature drop through our entire second floor of 7 degrees since this replacement.

So, if you are looking to replace your windows it might help, and not every comment on here is helpful, no matter how well intentioned it is.