I’m going to take a hard-line view on this but it may help you in the end.
By allowing some owners to park in visitor spaces, you are taking away common property and giving it to them. That ois of considerable value and those that don’t benefit are therefore being defrauded.
Also, when your plan was drawn up, the shire decided the community required a certain number of car spaces. Has that need changed in any significant way?
But let’s just assume that the shire says OK; you are legally obliged to find a way of compensating members of the strata scheme who don’t get any benefit from this.
The easiest way to do that is to RENT the car spaces to the owners who want them and let the money go into the strata scheme’s funds. That way everyone can benefit, the multi-car families can park their vehicles and the owners corporation doesn’t lose a valuable asset that it may require in the future.
Take that plan to the Shire and they might be more amenable, as will the nay-sayers when they realise their levies (fees) could be reduced accordingly.