"Reader analytics" is the term you're looking for. There have been several startups in this vein over the years, but only Jellybooks is still going. I don't know if they support the writing process, though; I've only seen them discuss completed books being tested. You should ask them.
I haven't seen an equivalent of a subcompact publisher (https://craigmod.com/journal/subcompact_publishing/) that provides reader analytics and supports authors as they write.