It’s always good advice to read all the comments before, a lot of the questions just repeat themselves . . . so one wonders. :-) Great guide, great additional info in the comments - thank you all!

I did the jump on the connector for the thermal sensor, as proposed on the blog entry by Eric. Put in a Sandisk SSD and even get a temperature reading for the SSD (which gives plausible and changing values). No fans running - thanks for the hints!

And btw, if you clone your old drive (e.g. with CCC) before onto the new one. Then do the swap - as a result, you just start up the computer with no issues at all.