The C2750 was my backup server in a dedicated and "homemade" enclosure and for sure the C3558 replacement from AsRock seems great, or there is one from supermicro as well (A2SDi-4C)... But I'm a bit cautious now... The C2750 was not cheap and didn't last long, I don't want to have again similar surprises... and the C3558 replacements are not cheap.Will you get another mini-itx board?
I also thought about changing the mainboard and eventually repurposing some old hardware lying around (xeon L5640) but I couldn't find a suitable (i.e. rather small size) mainboard.
So in the end, I'm considering using a X9SCM mainboard in a standard enclosure that I have. It'll be perfect for a backup solution and it's gonna be off site as well.