There needs to be another place he can have peace and quiet, if that is what he needs. 1 bathroom in a place with 3 people using it --> you do not get to park on the freeway during rush hour.
I find this interesting:
DH thinks that DD and I should just make sure to proactively use the bathroom before he goes in there ... if he’s planning on being a while, he needs to announce that and ask DD to use the bathroom before he goes in there - but that it’s his responsibility, not hers.
I agree with you. It's odd that the world should rearrange itself around him doing something that inconveniences others.
That said, I believe in giving introverts what they need to recharge. Is there a room, any room but the bathroom, where he can shut the door for peace and quiet in the morning?