@braillynn for mobile, Tasks.org's task app, Joplin for notes, and the Etar calendar app (or any app with DAV support). All three are on F-Droid and can plug into Nextcloud, which allows you to sync your tasks, due dates, and notebooks with some desktop equivalents. I use the desktop Joplin app plus the default Gnome tasks and calender apps, and it all works seemlessly.

@braillynn Personally, I've found that org-mode (an emacs extension) works great for scheduling