I think it is tiresome to complete a task in the current way: select a task and complete. It would be great if you had a checkbox for task completion instead of task selection.

This may cause a confusion among current users. But other major todo apps like Todoist, Wunderlist, Google Keep, etc. use a checkbox for completion, so I think some users and newbies find it more familiar than the current way.
Additionally, you already have another way to select multiple tasks, clicking with Ctrl or Shift key. I guess a checkbox for selection might be unnecessary.