Yeah I would say sunshine is like… vaguely more outdoors-y and direct from the sun, while sunlight could be twinkling through the curtains or what have you.
So I can at least see why it’s not a synonym in Wanikani’s system itself, since that might conjure more implications of springtime and picnics and things than the kanji does itself, but in terms of recall I think it’s A-OK to add it as a synonym.
Yep!
Look for this button:
