There’s already an international open source and federated standard for Journals, Tasks and Notes. It’s called iCalendar-standard (RFC-5545) . See the VToDo component, relevant for your Google Keep alternative.

Also check out JTX Board

• Fdroid
• Website

Pros:

• FOSS
• Federated

Cons:

• Markdown only, no WYSIWYG.

Syncing:

• WebDav server (many FOSS and selfhostable servers available). For mobile client there’s DavX.
• WebDav Encrypted sync server Etesync which is FOSS and selfhostable.