I think a better way to control how fast people go is thrust and mass.
A freighter has a lot of mass. it should be slow to accelerate, but still able to move quickly. A good way to punish players that go over your desire limits is a “physical stress” indicator or the like. Set it so the computer automatically stays under that limit, but if you want to play the odds you can go past it. make it scaling. 100% is safe. 105% is safe. 110% is 10% risk, 120% is 33%, 130% 66%, 140% you’re dead or flying apart. I like of the mechanic with asteroids being the way it’s damaged, with parts of the electronics and such also at risk because you’re pushing the electrical systems.