Is a repurposed AMD Ryzen 3600 a reasonable CPU for a home NAS?

Truenas sells systems that use 4 core atom processors. You’re 3600 is more than enough

Depending on the quality of the board (not all 550M's were built the same), it should be more than enough for a standard home NAS. I made the comment on the 550M as I've seen a few postings where SATA and NVME/M.2 bandwidth was not as hoped for due to design constraints to put in other features on said board vs a competitor's that may not have other features (more 550M standard design). So, unless you're going for insane 10+ simultaneous 4k streaming, large dataset modelling, local hosted gaming- it should be fine.

Also, be aware of a very small number of implementations being affected by a TrueNAS ZFS bug:
https://www.truenas.com/community/t...penzfs-ongoing-discussion-and-testing.114390/

All of that being said, the rig as described should be good for several more years of use as a NAS, provided you have the proper power supply for said board/processor, drives. You made no comment on video card, so that also would come into consideration if you were expecting your NAS to do transcoding for video streaming (not so much drive bandwidth, but power consumption and power supply requirements).

That setup should be good for many years as a NAS, with some low-use BHyve virtual machines thrown into the mix for good measure.

Depending on which calculator you use to find the specific PSU minimum for your rig, it appears 550w should be more than enough under load (by about 50w-100w) and should handle things well. As for using that video card in your rig- yes, it's overkill for NAS, but if you run any video streaming from it (such as using Plex, JellyFin, etc), it will happily allow you to run 3 transcode 4k streams (and it supports HEVC too) at the same time without issue. It sounds as described (without more specifics) as a solid and fast hardware build that should work well for many years.

I realize I'm coming to the party a little late here, but FWIW I have a pair of older FreeNAS builds and a newer TrueNAS build all running on 1ghz dual core AMD Turion procs. They aren't particularly robust, but they're fine for storing home pictures, a movie repository, my ISO library, backups, etc. Your CPU will be fine. Cheers, and good luck!

I have this setup as nas/docker server. Works great!