Considering the author already strips the DRM from the purchased e-books and also apparently knows about Z library, a simple solution seems to be the buy the e-book, ignore it and then download the same e-book from Z library.
I also feel the frustration though. I won't buy e-books unless it either has no DRM or if I know I can strip it easily afterwards.
Most of the "eBooks" from Z-lib and archive.org I've downloaded are nothing more than poor-quality scans of paper books converted to ePub via PDF+OCR. They're not even worth getting. The eBooks I rent via Overdrive or Kindle, though, were actually prepared specifically for that format.
The author usually gets <5% share from the book price. I guess it's much better (although not legal) to just skip buying the book and send a donation directly to the author.
I also feel the frustration though. I won't buy e-books unless it either has no DRM or if I know I can strip it easily afterwards.