second CP needs to be not hard to defend against mid if you have uber, but hard to defend against mid if you don't. spires are an easy way to make it hard to defend without an uber without making defending with an uber harder
if you have more than three routes between last and second, it's going to be impossible to make sure nobody's hiding in them when you push out of last, so don't do that
elaborating on the previous statement, if you have a ton of corners in all the rooms between last and second, that will dramatically slow down people who are checking for lurkers and pushing out of last will be slow
there needs to be a demo rollout with at least a lunchbox on the way to mid, add a pill if there's serious risk of taking fall damage
mid needs to be less detailed than other areas of the map because it will have 12 players on it at once at the beginning of every single game, and people will rail all on your map with no compassion if they lose the map after taking framedrops at the midfight
valk's guide is pretty good for more concrete advice.