There is a search solution for zipped fb2 files. Not exactly what you need, but it has potential.

The project has similar story to Anna's archive. There is 0.5 TB of archived books, and the project creates index of all the books with text, title and aruthor search capabilities, gives html UI for search and reading. On weak machine it takes about 2 hours to build that index.

So if you have zipped archives of fb2, you can use the project to create web UI with search for those files. Without need of enough space to unpack all the files.

You'll have to translate some russian though to get instructions on how to set it up.

https://gitlab.com/opennota/fb2index/-/blob/master/README.ru...