Banning the car door wont happen.Only problem I see with your design is if you do pull in a car, not to bang the tank with the car door.
Your RO unit, with a much bigger tank can also be in the garage. Plus a point as to where to flush old water when doing those water changes we hate. I'm using a 6m flexible hose to the drain outside.
If you keep gravity in mind you can move all your water using only one return pump.