What it boils down to is ALL motion we deal with in our 3D word is RELATIVE motion. BUT Inherent in ALL matter is an ABSOLUTE MOTION at a steady c/Sqr2 speed. in the direction of the 4th dimension---we call this motion TIME and it is the only REAL motion.
Take two cars, side by side, matching speed on the highway--at a steady 60 mph If their motion vector angles are parallel, then there is NO relative motion between the cars. But if one car veers off at an angle---there is relative motion between the cars from the driver's POV.--the greater the angle the faster the relative motion. Of course the relative motion cannot exceed 120 mph-at 180 degrees-as the "absolute" (highway) speed of each car is 60 (60+60=120).
The rocket is in the same situation---Its movement relative to an observer is a function of this Absolute motion and relative vector angles. The motion of the rocket is RELATIVE to the observer.--The greater the Time motion vector angle,( a function of Energy) the greater the relative speed.
But since there is only ONE Absolute speed, the relative speed between the observer and rocket cannot exceed a function of the absolute time vector speeds---In the 3D world this angle is 180 degrees like in the example of the cars. In the 4D relationship, the angle is 90 degrees, because past 90 degrees the rocket would be going BACK in time relative to the observer.----So Relative motion tops out at 90 degrees and the relative speed at this point is The speed of light.----SO while the rocket may still move its time motion vector angle, it would have lost its relationship to the observer past 90 degrees at light speed.
You can also see the principle of "time dilation" here too, when one car veers away, it DROPS BACK in distance on the highway too as it moves relative to the other car--as it is not changing highway speed, only direction--The greater the veer angle, the greater the relative speed as it also drops back. Relative speed tops out at 180 degrees at 120 mph. ---Not exceeding 120 mph
But with the rocket we are dealing with absolute motion in the 4th dimension that gives relative motion in the third. SO time slows down (or "Drops back") for the rocket relative to the observer, until 90 degrees is reached and stops altogether (restively speaking) while at light speed. Beyond 90 degrees the ship runs backwards in time relative to the observer SO there is a disconnect.
Ship time on the rocket may be only a few weeks, to the goal, while years may have passed on Earth ---those yeas being the light year distance to the goal.