kimbler parent
I do a similar thing with https://www.thateasy.me/ I use it to test the latest coding/automation tools. I think I prefer your design :)
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?
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).