30/1 voting rule exactly makes this attack vector easier.
But, in the meantime, I've found you have proposed very similar solution!, i.e. 1SP = 1 vote!
Dividing vote power would have the same effect as reducing the number of possible witnesses to be voted by one account to 1, but still allows to cast votes from one account to more than one candidate (smaller votes, though). So a more flexible solution. 30 candidates are too many, 10 much better, I would go for an even smaller number, 3 or 5.