Hello again,
following Ivans advice I ordered a CH341A programmer, which unfortunately did not arrive yet.
I continued my experiments with the Raspi though and I would like to share the results with you.
When I run flashrom with the spispeed setting set to 70000, it now finds a "Generic flash chip "unknown SPI chip (RDID)" (0 kB, SPI).". See attached log.
Any ideas on how to continue here? Or should I just wait for the programmer to arrive.
Thanks in advance!
Fabian