Not going to help this along, because I'm in the same boat. Running older gen dual Xeon on FreeNAS. Looking to make a change to unRaid for ease of use. Really looking forward to see how this thread develops.
My goals are the same, upgrade to newer gen hardware but prefer enterprise reliability. Looking to expand to 150TB+, 4K UHD, ~10+ concurrent streams / transcodes (I don't allow transcoding of 4K). Would like to explore HEVC h/x265 for space savings which pushes me towards a need for GPU transcoding.
Not really sure the best migration path for hardware, etc. while changing OS as well.