I forgot part of the plan for this: your near-lightspeed space ambulance would also be indestructible and have perfect inertial dampening. So to decelerate, you just try to crash into all the debris you were trying to avoid as you picked up speed. So it’d go:
- Accelerate away from planet – dodge debris!
- When you’ve gone far enough fast enough, decelerate – hit debris!
- Turn around.
- Accelerate towards planet – dodge debris!
- Crash into planet!