I’ll just comment on some parts of the topic.
Do you plan to deploy raidz1 with 3 drives? If so, it doesn’t look very optimal.
650w looks like overkill. Each HDD drive usually needs 10w (20w when starting). Enterprise SSD 10-25w (consumer SSDs usually need less). I believe you can calculate the rest parts yourself.
If you plan to use it for jellyfin transcoding, it seems like overkill, unless you would have many simultaneous clients (which need transcoding). Can’t say for AMD, but intel 12-14th gen iGPU can transcode 3-4 UHD streams at once.
IMO, you can save a lot by using non-ECC. Especially when you’re stuck with the m-ITX limitation. However, I highly recommend you take a look at this ECC vs non-ECC thread.