Since I'm still experimenting with kubernetes and docker and on the search for a good filesystem-backend, I can put the data on.
I first used a simple nfs-server then had problems with postgres, then I tried cephfs, which worked but seemed to be an overkill and I guess you need more power and memory to really use it effectively.
And now I switched back to NFS using (openmediavault)[https://www.openmediavault.org/].
OMV is based on armbian and provides all you need to create an NFS/Samba/ftp/...-System outofthebox for you. So I now have two rpi4b setup with it. One with an two mirror-raids once two 256gb-usb3-sticks on usb3 and two 256gb on usb2 and on the other on 2TB-usb3-HD which will be my file-backend. Privately and for docker/k8s.