This contradicts what you already said ("there are always more TODOs"). Which one should I reply to?
If your employment has fixed deadlines, and your employer does not react to efficiency increases by setting earlier deadlines, then you should expect the headline effect (efficiency increases mean your weekly hours decrease).
> I can finish it on Monday and have it ready for the Friday deadline and then spend my time how I wish OR I can report the task completed and then expect to have more tasks assigned
Are you serious? Do you think you just have free choice of whether to do more work or not? If you completed a task in 15 minutes and spent the rest of the week surfing do you think that would be allowed by your employer? Most employee contracts would forbid it.