I posted this in the "Its Brutal" thread already so this is mostly a copy-n-paste. This is my ship. An overweight brick (4k tons) that needs 26 epic thrusters to cruise at ~300 km/s (well 250km/s for most trips since they aren't long enough).

Made a combinator setup that checks if the current destination is either the inner belt or outer belt and the previous destination was NOT the inner belt or outer belt. Meaning we are heading towards the belt. So it starts throttling the thrusters to roughly 75 km/s (dumb velocity check).

This slows and keeps the ship decently slow by the time the belt is actually reached.
I also have a second combinator that further throttles the thrusters to about 25 km/s (again, dumb velocity check) when we are between the inner and outer belt stops.

So with my setup a trip such as Nauvis -> Gleba -> Inner belt -> Outer Belt -> Aquillo goes something like this for the speed:
Nauvis -> Gleba = Full-throttle.
Gleba -> Inner Belt = Slowed down.
Inner Belt -> Outer Belt = Snail's pace.
Outer Belt -> Aquillo = Full-throttle.
Works the same way in reverse. Aquillo -> Outer Belt is slowed down while inner -> Gleba is full-throttle.