I don't understand why you can't use libgen, which has quite an extensive collection. Why only Z-library?
Additionally, a huge thing for me that keeps me going back to Z-Library repeatedly is working fulltext search for the entire contents of their library. I have a number of things that I search for that get a trickle of new scholarly review every year, and Z-Library allows me to continually find new references within the body of text to published books. I have reviewed every alleged fulltext book search engine on earth that does not violate intellectual property laws, and nothing even comes close to the utility of Z-Library.