Export my own ePubs from Books app to PC with highlights?

Sorry if this is a stupid question, but I've been scouring the internet for hours with no luck at all. I had an ePub file saved to my Windows PC -- I did not purchase this file from Apple, this is a DRM-free file that originated from my personal library. I emailed it to myself, opened in the Books app on my iPad, read it and highlighted as I went, everything was great. This was a while ago, and I have since lost or deleted the original ePub file from my PC. Now, I want to send the ePub back to my PC (where it originally came from), and I want to keep the highlights so I can view them on my PC's ePub reader, too.

This does not sound like something that should be difficult to do, but I'm losing my mind over it. I can't share or export from Books, which I guess is to prevent stealing books bought from Apple Books. But as I said, this is a file that I already owned before I sent it to my iPad. I've tried digging through the Files app, through iTunes, no luck. I tried to use Calibre (which I used to use to convert ePub to mobi for my Kindle) but no luck. The closest thing I found was a roundabout way to zip and unzip the ePub file, but it only seems to work on Mac. I even got desperate and tried one of those semi-sketchy applications suggested on bad articles, and I was able to get the ePub file itself transferred in the correct format, but the highlights disappeared.

I thought at first that I was just not tech-savvy enough to figure it out, but I'm starting to think this is an Apple issue. They're so obsessed with their customers using only Apple products and software that they remove basic functions like sharing files. I'm about at my limit with Apple. This is a pretty new iPad, but I will not be replacing it with a new one, unless they get their s**t together. This is ridiculous. Either they've intentionally disabled this function, or they've made it so difficult that someone who grew up in the tech/digital age can't figure it out alone, even after hours on the internet.

Does anyone have any suggestions before I just throw the whole thing out the window and revert to the caveman days of pen-and-paper notes?