The past

[Darwinian]

I think you're mixing up Galilean relativity and SR. If I were on the spaceship, I would see the ship clock running normally and the Earth clock running slow. Remember, there is no priveleged reference frame with respect to which motion should be defined.

I probably am However, surely from your position on the ship you would see the Earth clock running quickly so that when you eventually returned to the Earth, you have hardly aged at all whilst your friends and family are much older.

Yes. Relativistic mass is a superfluous concept, and in my experience of explaining relativity, it tends to be more of a hindrance than a help. Mass doesn't change in motion, only the apparent mass as measured by an observer in motion with respect to the massive object.
To go back to our spaceship idea, if I were to measure my mass on the spaceship travelling at 0.5c (w.r.t Earth), it would be exactly the same as it would be if I were to measure it on Earth.

That's a relief then.