The chances of finding a world that would not kill the Boraal is miniscule. To find one in the time allotted during Homeward would be impossible.
This assumes that the Boraal star system is in the far off reaches of the galaxy and not in the interior of the Federation. If it is in the interior, then the surrounding star systems would be thoroughly surveyed, and the chore of selecting a suitable replacement world for the Boraalans would be no more involved than simply checking the records on the local neighborhood. The Vacca star system was a mere forty two hours away.
"There are countless M class planets in Federation space which can support the Boraalans
Half the population of Europe died
Yet when Amanda moved in with Sarek, half the population of Vulcan didn't croak.
What would they have been thoroughly surveyed for? To catalog it, or to determine if it's a possible place for a colony? Then if it is for a colony, a colony for who? Humans? Vulcans? Andorians?
Then is it possible to accurately determine what each species on the new planet, could do to its new inhabitants. The Boraal weren't going to be kept in a habitat like the Lunar colony (didn't that one female cadet on the Valiant tell Jake about coming from Luna and going outside the habitat with her father?) or the Mars Colony.
There are too many variables involved, too many possible out comes, only one in which the Boraal survive. Chances are far greater that they would die out on the new world, than survive. Yes there is a slim chance that the Boraal would beat the odds. I will admit "slim" is better than "no" but is it worth the risk to the other planet? No one seems to care about the life that already exists on their new home and if there is a life form on that planet that would grow to be intelligent in a few million years if not for the Boraal.
By the time of Amanda and Sarek, and indeed any interplanetary cooperation and commingling, the medical science would have been at a point where disease would no longer be a factor. What I mean is that each planets science teams would come up with medication or vaccines to protect the new comer, in this case Amanda from infecting the Vulcans and them from infecting her.