I chose Nextcloud as my first project because I had an interest in the project for a while. I did an old fashioned install which I later rebuilt with docker. I learned a lot doing it manually twice first. I echo the others. Find a project you like, preferably with its own community so you can ask for help when you inevitably mess something up.
I’ve never been able to fully transition away from the proprietary TickTick tasks. Nothing seems to have the features I’m accustomed too. Then again, I’m on a dysfunctional task non-management spree right now, so maybe when I get my shit together I’ll try again. For context, I use a modified version of the GTD strategy to keep track of my todos.
Before TickTick I used Astrid. When Astrid tasks was bought and killed by Yahoo, I thought they were over, although it seems there is a fork https://github.com/tasks/tasks (GPL 3.0) that also syncs with tasks.org. I haven’t done thorough testing yet to see what kind of issues I would have using this new Astrid and Nextcloud, but this is the best open solution I’ve been able to come up with and its been on my project shelf for over a year waiting to be tested.
For calendar, nextcloud synced with Thunderbird and a proprietary phone app (I know… I know) seems to work well for me. My partner uses icloud and it generally interoperates fine. I even have a raspberry pi in the living room that pulls in everyone’s calendar and overlays them as a “family calendar”