Proton Calendar had been an amazing alternative to the proprietary app I’d been using.

As far as I can tell, there is no ‘marking items off’ though.

All of my tasks are time specific, so this is no issue for me personally; when a task is complete, I just swipe the notification tray clear.

Proton Calendar does have a widget though.

If you’re just looking for a way to neatly list your tasks with the ability to separate pending tasks from completed tasks, I might recommend Joplin. I use it for notes, but I also use it for my shopping list. Using Markdown, you can add boxes in from of list items. I’m uncertain as to whether Joplin utilizes widgets, however.