How does altitude affect the TAS and thrust of an electric fan powered aircraft?
The Helios was an electric powered prop driven aircraft that achieved an altitude of over 96,000 feet and maintained it in level flight for more than 40 minutes.

as altitude increases

You maintain wing lift with constant IAS until the wing airflow approaches the speed of sound (TAS increases).  Shaft power output must increase to spin prop faster to maintain thrust or... changing prop pitch is helpful instead of increasing rpm.  This helps avoid the &quot;sound barrier&quot; for the prop.

as motor rpm increases

Even when not limited by supersonic prop tips, increased engine bearing friction from higher rpm requires a greater power draw.  Again, the ability to vary prop pitch is enormously helpful to maintain optimal prop AOA as TAS increases.

can I fly with the same amount of power at 50,000 feet as I can at sea level

You can get a lot closer by designing your aircraft to fly at 50,000 feet by:
 1.  Reducing wing loading by removing weight
 2.  Increasing wing aspect ratio and/or increasing wing area
 3.  Increasing the **area** of your prop so you can push more air at the same rpm.

With props, you must compensate for thinner air, but higher rpm is not the only way.