But if the earth were flat, the angle between the north star, Polaris, and the horizon, would always be the same, regardless of latitude.
Not at all.
mw is correct. Why is everyone in this thread so bad at geometry? It is trivial to show the curvature of the earth with basic geometry. The greeks did it over 2000 years ago.
There is no curvature to the earth.

I made a very long post somewhere in one of the 30 threads on this topic about conclusions that can and can't be drawn from horizon pictures like these. The long and the short of it is that to actual get measurable curvature*, you have to be at around 30,000 feet, have at least 60 degrees field of view, and have be careful to minimize lens distortion**. Those pictures prove nothing, and parroting statements while reposting them over and over isn't an argument that will convince a rational human being.
tl;dr - photographs of the "flat" horizon are rarely, if ever, useful in these arguments. Also pictures are easy to manipulate, so there's that to worry about as well.
* based on a earth with 4k mile radius, as per the "globe model".
** Lens distortion is a real issue, because lenses are round, so you can actually take a photograph of a flat line and have it be curved up, curved down, or flat, depending on how careful you are. Likewise, you can take a picture of a curved surface and distort that curvature so it isn't noticeable.
So you say. In fact, a 360 panoramic view from 10,000 ft should give a
clue. Still you say you don't get it. Ok. Let's try something different. Water in my glass, in my pool, in the lake behind my house. It's no more or less curved than the ocean. Why? Because water surface remains flat at rest. Water has never stuck to a curved outside surface of a ball, ever. Only heliocentric theorists believe such nonsense and then refrain from providing evidence for it. For Catholics, believing that is really not an option because heliocentrism has been condemned by the Church, by scripture, by saints, by popes. Perhaps a long consideration about the properties of water and the church's condemnation will clear things up for you.