Cold proofing is a great way to develop flavor, but the down side is that it’s hard to tell if something is ready to bake. It’s also easy to overproof for the same reason. It just requires baking the same recipe multiple times using the same fridge until you get a handle on the proper amount of proofing time. Another way you could develop the flavor is by using a preferment like a bigga.
Undercooking can be avoided by taking the temp of the bread using a thermometer. It should be 200F inside, minimum.
But yes, you can def save a bread that hasn’t come out quite right! Toasting is a great way to do that.