ECC is not a hard requirement for me, but if I can get it - I'll try to, as to me it makes sense for something that runs 24/7 and handles my personal data.
I have a very strong aversion to separating storage from my server. I just don't see why I need to route power and network to 2 small boxes (none of which would do what I need it to do on its own + considering very crappy room layouts in rented apartments) and then fiddle with network access, when 1 slightly bigger box would do what I need it to do. Some 7-8 years ago I've bought dirt cheap second-hand Huananzhi x79 with Xeon E5 and DDR3-ECC with some low profile NVIDIA GPU and it all still works now - and something like that would mostly be OK for me even now (except I left it in another country).
That said, it's possible a reasonably powerful NAS will be enough for me on its own?
I don't mind that, I'm actively looking for recommendations. I also got pointed to https://www.minisforum.com/products/n5-air, but that one is 500 with no RAM at all, which is quite hefty.
So far I'm leaning towards trying to assemble something generic from used components, though I'm not quite sure what to pick best, motherboard being the hardest one.
As for TrueNAS - I might check it out if I do get off the shelf NAS, but I don't imagine I'll be using the Web UI much, to be honest. And I know about Jellyfin, I'm sticking to Plex just because I already have a lifetime Plex Pass and I like the app on my TV. If they keep shoving "community" features down my throat though - I'll do the switch.