Step 0 is to secure that box, as routers are obvious targets, even before they have self-hosted data. There are some products based on RPi, NAS and router form factors.
> suspicious absence of products in this space
Earlier efforts:
Apache Wave (federated)
Chandler
Diaspora
FreedomBox
Microsoft Groove (p2p)
Urbit.org
Sandstorm.io
Active OSS projects include Proxmox (https://community-scripts.github.io/ProxmoxVE/), Paperless-NGX (docs), Immich (photos), NextCloud and others, https://github.com/awesome-selfhosted/awesome-selfhosted