In the end you have to decide what's the best move for you at this point in your life. If you feel really strongly that your game concept is something special and you think you could make it with the team you have now, maybe you should try and start your own company. This is extremely difficult though as warby said. It's very rare that a publisher or anyone else for that matter will give you the money up front to start your company, even if you've managed to create a playable demo.
I wouldn't expect the developer to let you guys maintain the rights to your game if they decide they want to hire you on to make it. They don't want to make you rich and famous, they want you to make the company as a whole rich and famous and it's not unreasonable for them to expect that since presumably they'd be investing a decent amount of time, money, and effort to bring you onboard.
I think that if you do get an offer to make your game for the studio you should at least make sure you have the potential to see profits from it.