When it comes to #fullTextSearch ... I recommend #recoll ... https://www.lesbonscomptes.com/recoll/
It does take some getting used to, but once you've configured it to fit your needs, it works quite nicely.
When you find yourself searching for scientific terms in the database of all the digital books and scientific publications you once downloaded ... and you find something ... you maybe find better results than on the web ... that's awesome.