There's an important caveat missing: "and thus much more likely to transmit it [if they are infected]." Presumably, the Rt changes with mass vaccination. Only those who are infected may be more likely to spread it, but there should be fewer infected to spread it to begin with. And that's before the possible difference in viral load, like you said. I'd rather have a fraction of the people spreading a fraction of the virus, even if it may get to a high risk person, than have that many more people spreading larger amounts of the virus.
Its hard to know the numbers at this point. If 50% are immune from becoming infected, and the other 50% spread it 3x as much (natural Rt is around 3), overall spread and prevalence may be higher with a vaccine than without. If the vaccine prevents infection 90% of the time, then you’re still ahead. The number may not also be black and white, it could be 30% are still susceptible after vaccination but only with a higher viral load, etc. (posted a thread above)
The uncertainty is a sufficient reason to avoid this path.
But doesn’t shedding peak pre-symptomatically?
In theory, yes. Asysomtopmatics definitely shed, those vaccinated likely shed less, but the million dollar question is how much less