The RETURN fuel will not be affected by the tank being higher; my huge 3rd tank auxilliary has been working thus for many many years.
I highly recommend dis-carding the electric tank-switching valve and using TWO 4-way manual valves to control which tank is being used.
I very-much prefer any auxilliary tank to be plumbed properly to the engine, with a DRAW and a RETURN line, whether incorporated via manual switching valves into a multi-tank system, or being used as an only tank.
I have witnessed many expensive scenarios from those using "gravity-flow" auxilliaries that feed directly into a stock tank.
Plumbing properly with DRAW and RETURN lines actually costs much less and is less headache than any of the other options.