There's still value in training even in 1G, so it stands to reason that there's value in training in 150G even if you've mastered 300G.
It's not THAT tricky. The 150G scene lays it all out. We know that Vegeta got used to it at a low level of power, and we know that Trunks who is in possession of a high level of power isn't used to it. But he can brute force the gravity impediment by powering up even more, to the point where his obscenely high power level makes up for his lack of experience walking around with significantly heavier limbs, organs, and so on.
It has to work this way. Saiyan toddlers were waddling about on Planet Vegeta with power levels below 100, most likely. They were just used to it. Put Kid Gohan on Planet Vegeta, and it'll throw him for a loop, just like the RoSaT did. But put Kid Goten on Planet Vegeta, and he won't notice it because he has so much power to make up for his inexperience.