Well of course there's a reality outside of you that you aren't creating much of. But how do you infer from this that it's necessarily produced by
a mind, as opposed to the different bits of external reality being produced by
many minds? In your own immediate experience, some of it is created by me, some by wiploc, some by Keith&Co., some by Kharakov, some by Tom Sawyer, ...
Look at it this way. We
already know from science (which as you point out is no less real in your radical idealism than it is in conventional materialism) that humans evolved from bacteria. If nothing exists but thought then bacteria are thought. Why would you assume the bacteria have to all be the thoughts of a single mind, as opposed to each bacterium being its own tiny little bit of thought?
How does that follow? Here, I'll think a thought, as an act of will. 3.114441111555559999992222222... That thought is still an effect, caused by my act of will, acting on a prior thought, 3.141592... There's still a separation between the cause and the effect, with my will serving as the intermediary -- and I could not have constructed that effect in my mind without first thinking the thought that preceded it. That's what "A causes B"
means: A comes before B and without A there would be no B.
Put it this way, if there is a God then all causes and all effects come down to His will. I guess He lets our small minds have a little bit of will (causation) built into them, so that we are not complete slaves to circumstance.
But as you say, you
guess. All it is is a guess: just one of the ways it might be. That speculation in no way follows from the premise that nothing exists but thought.