You could preload a year's(or 10 years) worth of sunrise sundown times and reduce the need for as many updates

Yes indeed that could be done. It is not especially difficult to calculate them either.

A side benefit of having the RPi call in is that the last state it put the curtains into can be seen on the web interface. Some family members are rather concerned that these curtains will never work correctly because of the poor experience we have had with the timer that is controlling them presently. Being able to see the time they last closed or opened might relieve some worry.

There is also the issue of opening at 8:30am and not at sunrise. My blind uncle's guide dog enjoys barking loudly at the children going to the school opposite...by 8:30am most of the children are safely installed. It is quite possible that the curtains might be required to open earlier during school holidays so keeping it flexible seems sensible.