You’re welcome! You are right, inline fields don’t support subfields unless there’s some syntax I’m unaware of. They do allow lists but the way where works with a list, we can’t just use contains() to find a substring from a list. I also thought perhaps using the When date as a key might work for YAML but that’s not supported either.
I can’t think of anyway to solve this except for using separate notes for each entry.
P.S. Have you considered just adding a GAO: [[2021-06-03]] to the note for the book/website?