She is certainly old enough if she is 1.5 years old. If you need to put weight on her then I would do it before breeding.
The way you give it is fine, but if you give them several slices if bread a couple times a day (like my cousin using bread in place of grain) they will get sick, as bread is like candy to them, too many carbs and sugars.... It will bind in their gut causing toxins to build up and therefore causing a sick goat... I feed mine bred as a treat, but I don't feed much. My cousin lost his whole herd, another cousin lost all but 2 sheep like this, and my uncle almost lost 2 cows... So, I'm very careful about how much I feed...I feed my goats bread every day. They each get about 2 slices of wheat or other heavy grain bread. Along with it they get hay and other vegetables and fruit every so often. What is so bad about feeding them bread?
Corn is bad for them too, as it messes with their stomach too much. Hay should be good enough, is it regular grass hay? That's what I feed mine. Have you checked for worms?? That may cause them to be thin too.Well can corn seeds make them fat (and hay )