Nextcloud has a nice Tasks app. Integrates also with the Calendar and the Deck Kanban app.

Edit: and it can be synced to a mobile phone as it uses the CalDAV standard.

This is at least as hard as CLI, but Emacs’ org-mode is a very capable to-do list. There are plenty of emacs plugins that add reminders and notifications, though getting these to work on mobile may be a pain.

There are plenty of FLOSS alternatives: https://alternativeto.net/software/microsoft-to-do/?license=opensource

final thoughts: sleek is an awesome foss to do app based on todo.txt https://github.com/ransome1/sleek