This might be an issue with opensearch.xml, which is a standard for how browsers recognise search engines.
See here:
https://github.com/hnhx/librex/blob/main/opensearch.xml.example
I don't know how you're hosting it, but when I was hosting LibreX, I had to make an opensearch.xml with the correct domain and bind mount it to the correct location. I don't exactly remember the details since I moved to Searxng.
Also, if you're not aware, LibreX was forked to LibreY, which is the updated repo.
Anna's Archive, Libgen, Mobilism, IRC (I use a self-hosted service called OpenBooks for this). I use Calibre for metadata sorting, plug Kindle in and move books that way and keep it on airplane mode.
Also, new Kindle jailbreak for <= 5.16.2.1.1 if anyone's interested. Managed to get KoReader on my 10th Gen Basic.