The type of rocket that pushes against itself as opposed to pushing against the atmosphere (rocket engine vs a jet engine basically).

I do think a throttle is going to be extremely hard though, you are riding an explosion directly, it's not like a car, there is nothing to put breaks on. Your only real option to go faster is make a bigger explosion, your best option to slow down is to turn around and fire engines opposite way, but gravity is free, fuel is not, and fuel adds mass to spaceship, so using a gravity well to slow or speed up saves a lot of money.

And only the main engine will be big enough to slow you down with any sort of speed (like 2 weeks or months or years), it's going to be equivalent to how long you spent speeding up.

Even if you use gravity and your engine you can only take so many g forces for so long. I think it's something like 1.3g you can handle for long periods? Outside of scifi 'inertial dampeners' you are stuck to a certain amount of acceleration or deceleration.