If you complete it sooner, your estimates were off. It’s fine to slow down if you sense you don’t need to work quite as hard as you thought to meet your deadlines.
In general, estimation is a system of shared deadlines. If you give your manager the ability to think of you as always meeting your deadlines, I guarantee they’ll be happy.
Sure, as long as they never find out that you've been padding your estimates or that the project everyone thought was going to take a week turned out to be much simpler than anticipated and you got in done on Monday and just played video games the rest of the week they'll be happy. There's even plenty of bad management out there, so in some places this might hold true for a long period of time (even a career!), but it's not because that's what your employer expected you to do, or what they think they are paying you for. And a good manager is going to figure out pretty quick when the end result of your 1-week tasks is a two line PR that's something is fishy.
* Disclaimer: I have absolutely had the tasks the boiled down to two lines, but finding the right two lines took a week. A good manager is going to have sufficient understanding of the work to tell the difference.
I don't think employees believe it either. After all, I notice your not asserting that you wrap up your work for the sprint, then tell your manager your knocking off for the next week since your work is done. If the expectation is just that you get your assigned work done, why wouldn't you? Why would you instead keep your manager in the dark?
Because you're already providing enough value to justify your wage even with padded estimates and slacking off (otherwise your manager wouldn't have been happy to begin with). They just want to get more out of you via the expectation of "reasonable best effort".
In other words, your 50% effort was already good enough to provide the company the value it needed (was willing to pay for), but some managers feel entitled to 100% if you demonstrate that capacity to them. So just don't.
And the reality is that many places don't genuinely need people's best. What they actually need is "good enough", and that's already reflected in the wage via profitable business outcomes that role contributes to. Time and effort have nothing to do with it.
To use your own words, I'm not sure where this idea came from. Any manager will absolutely expect you to tell them when you've completed your assigned tasks. No one wants the employee they have to keep checking in with to make sure they haven't completed their current assignment and are now sitting around staring at their cubicle wall instead of telling anyone.