I’m not quite sure what you’re unhappy with. Once you open an app in slide over mode then it remembers that and will open it in slide over mode the next time you start the app. If you go back to the home screen and then open the mail app it will open it in full screen mode. Then the next time you use command-tab to open mail it will open in full screen instead of slide over. Then you can just use command-tab to switch back to the original app
But if you want mail to open in slide over mode and you’re just looking for a keyboard shortcut to close it, then unfortunately I don’t believe there is one.