I have a 170hz for my main monitor and 60hz for my 2nd and it works, Just had to make sure my 170hz monitor was set to be my primary in the KDE display settings.
I think visually it might say the selected hz in my case 144. But I can tell that in practice it is not at 144. If I disable the other monitors only then does it actually go to 144. I’m not sure if I have a config issue, but I thought this was expected behavior from X11.
X11 does support multiple refresh rates. It's just that usually the compositor or window manager vsyncs every display, thus making everything refresh at the lowest refresh rate.
Are you using KDE? If yes, place these variables in /etc/environment and reboot:
From someone that was in the same pipeline and made the move, you won't regret it. I know it's early days for Wayland, but my experience on AMD and Intel has been very positive.