Actually, you’re right op.
Keep the piano, your work station wit two monitors, give the eleven month old the biggest room. Keep your living room and dining room. You deserve the space as you need it.
You kick that teenager out into the garden. Get an outside loo so you can lock him out at night,
Then maybe when your step daughters gets a little older, you can get them out there too, then you can have just you, your husband and your own biological kids in the house, they can have a bedroom each.
Go for it. Just explain it to social services when they come calling because you locked at thirteen year old out the house each night and made them sleep in the garden and use an outside loo. They will understand it totally. Neither you or your husband will be done for neglect. You won’t risk loosing your other kids. Not at all. They will totally understand you need the space for your monitors, piano, dining room and lounge and why they had to be locked out at night.
They might even shake your hand. Suggest other parents lock their kids out the house at night. Get outside loos.