What are the current procedures for applying for a Covid-Extension in Bangkok for a TR-60 Visa?

You understand right. At CW, usually, ask you to apply for 30days tourism extension first. This will be added to your current admitted stay until stamp, so you wouldn't lose any day from your current stamp nor from your 30days extension. IF you apply for Covid extension at CW for the first time, that will start on the day of your application, not added to your current stamp.

>Do I need to apply for a 30-Day-Extension of my TR-Visa before I can apply for a Covid-Extension?

Almost always yes.

>As I understand from what I have read in the group so far: If I apply for a Covid-Extension for the first time, the “Covid-Extension-Days” would be added to the date when I applied and not to the date when my Visa runs out?

For CW, you will have to apply for a 30 day extension of stay based on tourism. This extension of stay is added to your existing admitted to stamp.

>When applying for a Covid-Extension, I will first get an “Under Consideration”-Stamp and will have to come back later? What date will that approximately be? In one week, two weeks or some other time?

The last day to apply for a 60 day extension of stay based on unable to travel due to COVID-19 is May 28. You will get an "under consideration" stamp. The length of the stamp depends on how busy the immigration office is. Generally it's been about 2 weeks.