Until you understand how the rules of perspective work any further discussion on this is moot.
If you're wondering about what defines up and down it's the electric field between the dome and the ground. The question of outer space is simple, there is no outer space beyond the dome is water.
That's it over, believe in what you want and let me believe in what I want discussion over.
And please edit your main post from "How as an individual can I know if the Earth is a sphere or a flat disc? What experiment can I do that doesn't involve trusting information from a 3rd party that would prove what the geometry really is?" to "I believe that the earth is flat and any proof that you give me I am going to disprove it or disregard it"
The only reason I tried to prove that the earth was not flat was because I thought that you genuinely wanted to know, but instead the case is you already believe that the earth is flat and disregard any other facts that prove otherwise, like any other flat earther. In order to understand the proof given by any normal person you need to have basic intelligence which I think you lack and I no longer have the time nor the crayons to explain the concepts to you.