How are you planning to setup the drives? Most zfs articles I have read say not to go past 12 drives in a single vdev. Are you going to do two vdevs of 12 drives raidz2 or something else?

I have been browsing the truenas forums a bit as I work on my build. The Ryzen were never officially meant to be used with ECC but it works. If you go with Intel core processors you will need a w680 chipset for official ECC support. Most people in the forums with your kind of setup seem to recommend a XEON E-2xxx processor of some kind that is used. You can pair that with ddr4 ECC memory which is easier to come by these days.

I've nearly got all my hardware for my build and am just waiting on whether to pull the trigger on some Seagate Exos 20TB drives.