I nave always user Obsidianfor my notes (wich are mainly for D&D and personal project, not anything work or school related) and I’m extremely happy with it. It works completely offline, you can make CSS snippets yourself if you know how to, it’s very customisable and you could even avoid making an account for the cloud sync using other tools (I use synchthing). It doesn’t use LLMs in any way, it doesn’t has any ADs and it’s also free

I’ve been using Etar for years. Can’t complain. Edit to add that it’s an open source calendar app available on F-Droid.