I just wanted to give an update for other's benefit: I've been doing some research it appears that BookFusion (
https://www.bookfusion.com ) does exactly what I've described -- you can make Highlights while offline, then close the app, and then whenever you go online the cached updates sync automatically in the background. It's pretty cool!
The BookFusion reading app itself is pretty limited/basic unfortunately (especially in comparison to Moon+ Reader, which I feel like is one of the "gold standards" among reading apps), but I'm hopeful they'll be improving it. Currently they need to add the ability to customize the default Highlight color (as the current color is invisible on Eink), and the option to disable page turn animations (for eink).
I still welcome any other recommendations, suggestions, etc. regarding my original query.