I’m also playing with how to effectively get .mobi formatted books and highlights I create on my iPad into Obsidian. One trick, if you are willing to use Readwise is that if you use the send to kindle app to send your personal documents to Amazon and then to your device then the highlights on those appear on the kindle highlights page.
Readwise pro offers markdown support.
OTOH then you are giving Readwise and Amazon info on your personal books.
I REALLY want a system that allows me to keep all my stuff contained on my own WebDAV server.