Either that or the Sun-Earth distance changes during the year ...
Why do you assume this is not possible under a flat earth?
First of all the Flat-Earth Sun has to move around The North Celestial Pole in an "orbit" which has no apparent mechanism behind it ...
And now the Flat-Earth Sun also has to change altitude through the year too, again without an apparent causative mechanism.
I am just pointing out that an improbable situation is now even less probable ...
RET, of course, invokes gravitation - a force which has been demonstrated in the laboratory - as the causative factor in the relative motion of the Earth and Sun. RET also explains the change in size of The Sun by having The Earth in an elliptical, not spherical orbit with a distinct apogee and perigee.
Edit:
RET can also explain the phenomenon of 24 hour sunlight in the Arctic and Antarctic.
RET can also accurately explain the bearings of sunset and sunrise, and sunsrise/sunset being on The Horizon without the need for "bendy light".
In short, RET is far simpler and is, therefore, to be preferred.