With the mixer on top you can open and close the mixer remaining the main window intact.
That was the OP’s problem or am I missing something?
Also with a bit of effort you can very easy manage a lot of things with keycommands.
Open close editores, mixers and devices.
Close all plugin’s with one button.
Create dedicated workspaces.
You can work lightning fast and maintain your workspacesize the way you want.
That plus I recommend a dual monitor set up, Cubase is just not for one screen anymore if you handle large projects in my opinion.