Well, I’m not an expert either. But I have read John Wheeler’s Gravitation book, and I understand enough differential geometry to get though most of it.
You might not like this argument, and I bet there are better ones, but relativity is a very simple elegant model that predicted many things before they could be verified, or before anyone even thought to see them. One prediction is that if you take two synchronized atomic clocks, put one on the roof of a skyscraper and one in the basement, the one in the basement will run slower because it’s closer to the Earth’s gravitation. Well, sure enough, after a year, the basement clock was a few microseconds slower. So they swapped the clocks, and after another year, the two clocks were synchronized again—at least within the error of atomic accuracy.
GPS is so incredibly accurate because the position calculations take the actual speed of the satellites into consideration when measuring time. Clocks that move run slower.
Relativity explains why these observations exist via geometry. The geometry will not allow things that move slower than the speed of light to surpass the speed of light. Also interesting, it will not allow things that move faster than the speed of light to move slower.
Ultimately, of you want to go faster than the speed of light, you have to accelerate. This requires applying a force. That force has to move in the opposite direction faster than you want to go. Notice, I said force, and not any particular mass. Yes, big planets can accelerate slow probes. This is why jets that go 600 MPH expel exhaust faster than 600 MPH. Since we know of no force that goes faster than light, we have nothing to use to accelerate us faster than light.
What if the exhaust gases are coming from an object that in relation to the exhust gases is sitting still? The only thing that slows the jet down is drag. If there is no drag in realation to the jet or rocket the exhaust gases will alway EXIT and the same speed.