I finally found some time to make some progress on my NAS. I removed all four drives from the bottom of the box and started the FreeNAS installation again. Everything went fine, the system installed successfully and after rebooting and inserting all the hard drives, it recognised them correctly. There was a problem setting up the samba share, but it all worked out in the end. The Wireguard connection was set up very quickly and worked on the first try. I tried writing about 20 gigabytes and everything worked fine. The transfer speeds are not very fast (around 2mb/s), but I think the bottleneck is my router, which is very old. However, I think this speed is enough for me and it will cover my needs perfectly. I am still thinking about adding more RAM. The recommendation is 8GB, but I only have 4GB. However, everything works fine and there is still about 1GB of RAM available when I am writing to or reading from the hard drive. Maybe I will decide later if I find out that I "definitely" need to run more services on this server.
Jan "Jenkings" Škoda - blog
Simplifying IT, Amplifying Knowledge