Hi Jane
Wow, you are a busy bee.
You could do that, but you may have to add baking powder/soda along with the milk to get the acid/alkali reaction. By adding the buttermilk, you’re adding flavour and moisture, but you’re messing with the scientific reaction when it hits the oven and starts making all the lovely air bubbles to rise.
Buttermilk is acidic, so it needs an alkali to fluff it up, so to speak. If you use it on its own, it dampens the rise of the cake because it’s not reacting with anything.
For moisture, you might be better off adding about 1½ to 2 tablespoons of milk or adding stock syrup to it when it comes out of the oven. The stock syrup absorbs into the cake and adds extra moisture to it when baked/prepped.
Hope this helps.
Kind regards
Angie x