I hadn’t realised TrueNAS had stepped away from a FreeBSD base this year, to Debian… 
For server use, FreeBSD is where my heart is. I too have a Synology, and I’ve often toyed with the idea of building my own NAS box. I’d probably just run minimal FreeBSD on it and set things up myself.
I understand you were seeking a “hands off” solution, hence TrueNAS 