Alright, I got everything working. However it is all done in the ESP-idf from Espressif (and not the Arduino core). Basically i copied the example of the LAN8720 and removed all the setup. (so no internal registers got changed, all stayed on default), and changed all the names to from LAN870 to KSZ8...

I have just received the PCB for testing. I will let you know if i get some results.
(in case you or someone else might need it)

Hello Bo,
Soon i will be trying to do the same thing as you tried to do 4 months ago.
I wonder if you have had any progress or hints for me to keep in mind.

Same goes the other way around. If i will manage to get the PHY working i will let you know!

Many thanks,
Stefan