Colin is the MVP here, thankyou so much Colin!
So in summary, I needed to perform 5 actions as follows:
1 - In IIS settings, under Application Request Routing > Proxy settings, uncheck the box “Reverse rewrite in response headers”
2 - In IIS settings, under URL Rewrite > View Server Variables, add HTTP_X_FORWARDED_PROTO as an allowed server variable.
3 - In IIS settings, under URL Rewrite > Edit inbound rule, add HTTP_X_FORWARDED_PROTO for https (I did this in addition to X_FORWARDED_PROTO).
4 - In Sonarqube, I needed to go to Admin > Configuration > General and then scroll down to the “Server base URL” setting and set it to https://mydomain.com
5 - On the sonarqube server’s cmd prompt, run the command ‘%windir%\system32\inetsrv\appcmd.exe set config -section:system.webServer/proxy -preserveHostHeader:true /commit:apphost’