I'm still in the world and character building phase. Sketching landscapes, rooms and such, writing little short stories to form the characters (my idea is that they have a "memory", don't know if it works in the end).
As for now I'm completely analog, I bought a nice notebook and do everything in it. But when time comes I think I will go with Obsidian as I already use it for my other notes/projects. I think it will be very powerful with the graph-view regarding relationships etc.
Obsidian is not FOSS though, and there are alternatives. I just hadn't the time to look at them.
Edit: Obsidian can be used for free when you do not use their sync features. I use it that way and just have it in a Git-Repo for accessing it on different devices. Obsidian notes are in Markdown so you also do not have some kind of vendor lock, it's just about the links between the notes.
I bought a Fairphone 3 and put LineageOS on it a few years back and can recommend it. LineageOS is less secure than GrapheneOS as far as I can tell but the privacy aspect is there, as you have a completely degoogled phone. I have some friends that have the same setup on the newer Fairphones and they are also very happy and have a smoother experience than me, because it is a newer phone.
Fairphone is an European country that has a move to open source (https://www.fairphone.com/en/open-source/).
To the compatibility and functionality:
In general you will find an replacement for every app you now use that is from a big company. Open Source came a long way and most alternatives are even better in my opinion.