I find it odd that USA still uses and sometimes require to swipe the card and sign the paper when in Brazil even escorts have cc machines with contact less tech. Some are very small and uses the seller phone for Internet.

Speed of light is the speed of causality. And you are trying to separate Time from Space. It is not 'time' that is related to gravity, it is the 'SpaceTime fabric'. It is one thing

300 years passed on Earth, on the spaceship only 80 years have passed. It gets funky so you just need to read from bunch of sources till it click.

For example, everything moves at C on the spacetime continuum. Since we are moving very slowly through space, it means most of our speed happens on the time part of the spacetime. Since light moves at speed of C, from the POV of the photon, it has no travel time, it just teleports from where it is emitted to where it hits something.

In other world, if we surpass all limits and build a ship that moves at 1C. Whoever is piloting it would never know when to brake because there would be no time flowing.

Edit: break > brake

The experience is the same. Say you cut 2 potatos a minute on Earth. You go embark on some spaceship. You still cut 2 potatos a minute there.

It is not that " time is moving slower, I can do more stuff" like in movies, where some speedster see stuff in slow motion but can still act on normal speed.

It is time is moving slower, it also affects me, im also moving slower. In the end whoever is on the spaceship will experience the same time.

In another way, if someone is born, lives and die on a ship that is moving half the speed of light, for earthlings it may appear to them he lived like 300 years, but for the person that lived on that spaceship, he lived only 80 years just like everybody else.