Keep it as simple as possible to start with and then expand if you feel there is a need. No need for two servers. The first thing you should buy though is a book about basic network design and security if you're not familiar with it. It may feel like overkill now but future you will thank yourself.
As for the family in the same house... I try to make it as seamless as possible. Sometimes there is no need to tell them because the new service I set up integrates nicely with our devices. And sometimes I tell them "we have X now which does this, you can go to http://x.y.z.lan or use the app".
Some random and probably not very popular opinions.
- I would check out eBay for used parts and find a decent motherboard with an IPMI interface. Find an appropriate case with good reviews if you're not gonna mount your server in a rack.
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- Install FreeBSD and use zfs in a raid configuration that fits you if you care about your data. Run all your services in separate jails and be done with it. You don't need Docker anyway for your private NAS.
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- Plex, Jellyfin, the *arr stack and a few other resource hogging services have a nice web ui but that's about it. Do you really, really, need transcoding and all the features? Set up NFS, DLNA or whatever and use whatever client you like. I can't access my Jellyfin library right now because the backend is apparently too old (wtf). Meanwhile VLC and a few other more fancy frontend clients with cover art, ratings and stuff works just fine accessing the same files over NFS. You don't have to run some convoluted bullshit and reinvent the wheel every time just because you can.
No, the FreeBSD version from the ports tree. Equivalent to installing a package in a Linux distribution from the built-in repo's.
I rarely use it anyway and I could just upgrade, but enforcing an update by disabling all functionality still feels a bit excessive. Makes me wonder if any other artificial restrictions have been imposed.