This post was written several months ago and in the meantime I have given up on trying to cobble something together using existing solutions because they don't provide the extensibility that I want.
I'm building my own replacement from scratch focussing on tasks/projects/calendars first. The architecture is a postgres db exposing CRUD API endpoints and all changes are broadcasted over MQTT so I can easily hook into everything for automation and extension.
I have a desktop application in JavaFX and a mobile Android app so I can use Kotlin as one language across all clients and backend. Lots of code sharing going on for things like API models.