His physical build didn’t seem too bad, but yes I think you are going to have to install much of your additional packages building from source rather than just grabbing an executable. Probably less of an issue for a software dev who likes to chase cryptic build messages. There seem to be a number of ARM Linux distros around now, but he seemed to get stuck with his original choice of Rocky Linux;
I don’t know what the current status of that effort is, but you would probably find yourself managing ZFS and the containers without a GUI. Which is perfectly fine for some people.
The HL15 officially supports only ATX boards (9.6 inches deep). Be careful if choosing a “deep ATX” board. There is an additional inch or so in the HL15 before you hit the fans, but depending on the board layout, any headers at the front of the board (power, SATA, USB, etc) may be blocked by the fans with XL ATX boards. That Gigabyte board layout looks compatible, just a caution.
If that is your fantasy board, though, I guess I’m confused about your use case, which I thought was “[Storage server for 20TB available], which will serve SMB and AFP and host Plex and Home Assistant via Docker. […] I don’t need full virtualization.”
Seems to be overkill for that use case.
Not TrueNAS, ZFS. But that’s a complicated topic about how much RAM you really need for ZFS; it really depends on your workload.