As far as I know, Rocket R58 does not need any flush except maybe some warm flush after being idle a long time (as you noticed). In the R58 owner manual, p. 3, you also have a table with the equivalency between PID display temperatures and group temperatures measured with a SCASE thermofilter.

Beware that it need to be warmed more than 30 minutes before being ready for service.

Note : I'm not a R58 owner but a small coffee shop not far from my home use a casual R58. I'm always amazed by the heavenly extractions that machine is capable of ! I find these extractions on par with what they obtain on a LM (in another of their shop).

Due to the variance of temperatures that different roasts like, I don't care what the literal temperature is compared to the taste of the shot. I use appearance only when I'm chasing a taste issue.

I used Eric's thermometer exactly as you have - figure out a consistent process to keep temps in your goal range.

If only this hobby was simply a matter of dialing in the temperature at say 203F (pick the temp) and that would guarantee I could enjoy a god shot every morning. It's curious to me that there is no mention of sour/bitter, color in what you're looking for/dealing with.