Gotify. Fast. Easy. Private. The messages live on my server.

Also, there is a Gotify app in F-droid to get notifications on the go.

I have a use case where I use Next Explorer to share some directories with photos in them for others to see online. Clean, simple UI and no database, it just indexes your directories. PWA works well on phones.

It does run on docker though (like many, many foss apps). 🤷