Rather than getting into the financial weeds on this, and spending more money to work out how much you should be spending, I would just try to establish whether the majority of owners think the proposal is fair and the way to do that it to have a general meeting or even just an online vote.
I have known owners who have demanded a complete bathroom renovation because their water proofing failed, the repairs meant their tiles would no longer match and removing the other tiles meant their vanity, sink and WC had to be replaced.
The owner in this case seems to be acting quite reasonably so I would cut your losses and go with their plan or a variation of it.