I actually did a very similar build in an HL8 chassis earlier this year. It’s also running TrueNAS as my offsite NAS backup. 
I’m a big fan of the AsRock Rack X570D4I-2T motherboard. It’s worked really well for me with 10G networking, four slots of ECC memory support, and integrated BMC provides management features plus onboard VGA graphics. I can also confirm the 4x4x4x4 option is available in the latest 2.5 BIOS in my setup.
One note for you, you’ll need to remove the CPU back plate from the motherboard in order to install the fan you bought. I documented doing that midway through the comments. It’s easily doable with a little bit of patience.
I’m looking forward to your findings with 5650GE ECC support. You’ll see my in build post from above that there was also a lot of speculation on which CPU’s would or would not support ECC. Since it’s a PRO CPU, I think it will support ECC as well as integrated Radeon graphics.