You also change the rate at which you move forward in time depending on the local gravity and speed.

It is technically possible to accelerate so much you can travel forward in time at a nearly infinite rate.

You just can't go backwards in time.

Yes. Exactly.

Tenet and Primer both use the exact same time travel mechanic.
Step into the machine and you start traveling into the past at a rate of 1 sec per sec. You're traveling through time at the same rate as normal, just the opposite direction.