>  It needs to point straight forward and be level

Well, but if you do that, you will see a "flat" horizon whether or not the Earth is a sphere or flat. This will be the case for any round object no matter how high you go until, eventually, it all disappears from view.

Why? Because the horizon line you see against the blackness represents the points furthest away from you. If the Earth is indeed round, all of these points will be equally far away from you and also equally far beneath you. In order to see curvature to either side, the centre of your view would have to be higher or further away. WHich is not the case when you stare directly at the horizon. So no, the fact that the horizon is a straight line is not evidence against a round Earth nor evidence for a flat one.

What is, however, quite good evidence for a round Earth is the fact that no matter where you send the balloon on Earth, it always provides you with the same circular view. Meaning you always see as far in all directions 360 degrees around you as you turn the camera, and the increase in viewing distance always scales with the assumed rate of curvature no matter where you are in the world. This would require a very good explanation for a flat earth model to be supported.

Anyways, I don't expect this will convince you of anything, but I hope that you now understand that the level view of the horizon is not evidence against a round Earth. It  is actually exactly what you would expect to see if the Earth was round.