I was wondering about migrating to nextcloud, but I think I'd end up in the same place as you. WebDAV and SFTP are pretty powerful.

I use radicale for the webDAV server and it manages my calendar and tasks.org tasks.

For my personal notes, I actually just use text files and Markor on Android and sync then back home just using SFTP. It has a quicknote file and it's just one really long file of little notes I write. It's good enough for my really light usage. Otherwise I do pen and paper.