… You can travel to the location you originally perceived as the horizon line, and confirm that the Sun doesn't literally sink there.
Are you saying that the location of the horizon is a finite distance away? That assumes a globe-earth model.
What I actually said was that
if the earth is flat, then there exists a (geometrical) plane defined by its flat surface. That plane is infinite in extent. The FE claim (look up the wiki) is that all known celestial objects lie above that plane. (Where ‘above’ means, the side on which we live).
But what we see at sunset is the sun disappearing below the horizon line. This is consistent with globe earth, where the horizon is a small circle lying on a plane which the sun really does sink below. For an observer further West there is a different small circle, of course, so it is perfectly possible that the Sun disappears below my horizon, but remains above theirs.
But the same observation is difficult to reconcile with FE, for obvious reasons, hence FE requires some theory to explain the observation of the sun (apparently) disappearing below the horizon. (Which leads to the question, how does Zeteticism deal with the distinction between appearance and reality?)