At last the server is ready. Exchanged the mainboard with the SuperMicro board. Although it is now running "only" 16GB of RAM, it is ECC memory... So no more risk is broken ZFS pools.
So I made a mistake. And a big one. I jumped the gun with my choices for the motherboard, cpu & memory. My original post has a nice list for a very nice PC, not one that is really good for this purpose.
The use of ECC capable memory, cpu and motherboard is paramount for a ZFS based storage solution. So what to do. Already purchased these components and are nice mid-range specs.
Over 30 years ago I started with "playing" with a Mac in the role of server. The first steps into what turned out to be part of my profession.
All these time I have had administrator access to one or more server(s) running on the Apple platform. However the time has come to say goodbye to the MacOS and the Apple platform as a server platform.