Thanks so much!
Running into some issues with the Google Docs export though. Seems to not output spec compliant epubs (due to external fonts). The external fonts freak Apple Books out (the book fails to load correctly), Kindle seems to just ignore the custom fonts all together (the book is otherwise fine).
I got it working in Apple Books by first importing into Kindle Previewer and then converting the 'optimised' KPF to EPUB via Calibre.
https://mega.nz/file/439VlZga#ZLMNcDXHGrSUB4HivYFCy50jLkqSR5uTsA-nhqozgIg