Hi thikade
Glad that it worked for you. I was also a bit desperate until I got the example from Red Hat 
The file I published was the example file I got from Red Hat. In my own configuration I have made the same setup as you did (with a Grafana SA) in regard to Prometheus/Thanos, but I think you are right in regard to their use of the openshift-monitoring namespace. I also run Grafana in a separate namespace as you do.
Best Regards
Erik