I appreciate your trying to help with this issue. However, I already have that version of the refreshrate service installed.
Despite this, I did uninstall this software, restart, reinstall, restart again, then launch Armoury Crate.
All this did was to make the panel overdrive option appear; it did not solve the issue of being able to switch refresh rates.
In the meantime, there was just very recently an update from Myasus to the Smart Display Control (1.7.10), which is supposed to be due a bug with Windows 11, but actually reinstated the option to switch resolution modes on my Windows 10 setup.