Better to keep it short and sweet, I think. You don't need to make a full critique covering every aspect.
The crucial point is that this is bad legislation which enables rapists.
It is entirely predictable that predators will exploit it to identify into women's spaces and that this will lead to an increase in sexual and other assaults.
The other crucial point to make is that you will never vote for them again if they support the bill tomorrow.
(I think it's unlikely MSPs will be reading their email much tomorrow. It would be better to phone them if possible, though it's also not likely you'll be able to get hold of them personally. I don't know whether their office numbers are public.)