“The distance changes very slowly at each end point, and most quickly at the solstices in the middle. The towers would have to be spaced accordingly to make up for this.”
They also observed the moon. Making note of where the sun sets on a full moon would accurately give you the next monthly tower location in between those end towers.
Something to consider is the time frame. These were probably the accrued observations and knowledge of several generations passed down. They didn’t just go out with a slide rule and build these in the correct places in just a few weeks. They had time to observe and adjust until more accurate before building them permanent.
That’s a good point about the moon. It would also explain why they had both east and west observation points - a full moon rises at sunset, and sets at sunrise, and a new moon does the opposite.