Robert,
I have a few hardkernels and I was the one that got Centos-arm working on them and booting completely off the HD. Problem comes back to it is ARM. I have their Odroid HC4 for doing RAID, but could not get any Linux but theirs installed.
The HC4 should be running with https://www.armbian.com/odroid-hc4/, although this would be Debian based.
And as I mentioned, iRedMail does not support ARM. So I am looking for something x64ish.
the H3+, just like the complete "H" series is Intel based. The H3+ uses a "Intel(R) Pentium(R) Silver N6005" CPU. I have it running with RHEL9
Michael