There is no atmosphere. If the shuttle has no thrust, and you jump out of it, the speed you hit the ground will be basically the same regardless of when you jump (or, if you don't jump at all).
The shuttle is trying to land. When in orbit, it is flying around the moon, mostly horizontal to the surface, at a very fast speed. The Lunar Gateway does this somewhat quirky orbit (from the perspective of Earth orbits) that is elongated and not at all circular, as the gravitational attraction of the Earth distorts lunar orbits.
Regardless, your shuttle is landing. We get fictional control over HOW it lands in order to make the story work in a way that makes sense.
I think your best bet is to have the shuttle shed its horizontal velocity first, and land "strait down". This can be described as a matter of near-lunar flight path control: landing shuttles from orbit show up as descending dots, not as lines flying over the surface.
They then fall towards the surface, decelerating to keep velocities reasonable as they go. The lower altitude they thrust the more efficient it is, but also the less safe it is, and if your rocket has constrained thrust rates you might not have time near the surface.
As the shuttle lowers itself, it can have partial engine failure. It no longer has enough thrust to come to a halt before crashing into the ground.
So it could be a matter of working out how much remaining thrust it has, how hard the remaining thrusters can burn, how fast fuel is leaking, and the like to determine the "ideal" time to bail out.
Your space suits may also have small, weak EVA thrusters.
A problem here is that the space shuttle will end up impacting at about the same velocity. And in most impacts, having a shuttle crumple zone around you will increase your chance of survival. For you to impact slower than the shuttle, you need to have more rocket delta-v than the shuttle does when you separate. This is tricky, unless your EVA thrusters are quite strong, or you have something else like that available.
Lunar surface gravity is 1.6 m/s^2 -- and at speeds much over 20 m/s^2 you aren't going to survive. This means you have to bail out under 13 seconds before impact in almost every situation; if you bail out of a stationary object and take more than 13 seconds to hit the ground on the moon, you are really likely going to die.
A final idea would be to take aim for something on the moon that would help you not die. Imagine if there is a plastic membrane surrounding a garden - your plan is to jump, blast a hole in it, then use the expanding air to cushion your fall.
Doing so perfectly would be insanely hard, and missing by a bit would result in you being thrown into orbit (which ... might be better than splatting! You can be rescued from orbit.)
1/3 of an Earth Atmosphere is 30 kPa. If you are hit by 30 kPa for 5 seconds, a 1 m^2 human would be flying at 1 km/s (and have died from the impact). But the point is, there is more than enough oomph in near-earth-atmosphere air to throw a falling astronaut into lunar orbit.
So I'm imagining a precision jump that makes you fly right above a dome, blowing the dome to get a shockwave of air, then using that shockwave of air to knock you back into lunar orbit.
Those that jump early or late ... miss the key moment.