I've been wanting to create a similar holiday optimizer tool myself, but what you've done is marvelous! Do you take requests for new countries? I see that you're using https://date.nager.at/ as the source and country is listed, so perhaps it's easily doable?
I've added more countries now, feel free to see if it works for you :)
Interesting side effect of having to choose calendar years is that it misses the break between Xmas and New Years' Day that is often a good-value time to take off (and many people would take off anyway).