I use youtrack. It’s a project management tool. It’s not open source, but does have a self hostable option.

https://www.jetbrains.com/help/youtrack/server/installation-and-upgrade.html

It’s not exactly what I think you’re looking for, but depending on what you are trying to do, maybe look at hackmd/codimd.

It’s more like Google docs meets markdown formatting. It’s goal is realtime collaboration but I’ve definitely used it for syncing todo lists with people.

Codimd is the self hostable version.

Oh, and I think there is a way to have it sync with a GitHub repo too, in case that is useful.

Links for convenience:

https://hackmd.io/#

https://github.com/hackmdio/codimd

My brother also has his own NAS at his house. We sync our media between both of our servers to both share it and to serve as an off-site backup.

Everything else on my nas gets backed up to a cloud provider.

Like you said, it could be replaced it’d just be inconvenient, and media is kinda bulky so cloud storage for all of it would get a little pricy.