https://getpolarized.io/ seems like it's in the same space - it's a product I wanted to love, but was a bit clunky to use and didn't end up sticking in my workflow.
Would love to know what about the flow was clunky and why it didn't fit your workflow. Might be a lot that I can learn from that.
For a while I was trying to sync a Zotero library (since I need citation management when writing papers), and do all my academic reading (which is mostly pdf's) in Polar, but it was just a bit too much overhead to stick with.
Thanks for sharing your story, it helps to put things in perspective. I wonder how much they've improved since you used them though. The website looks very sleek.