Are people still using Sphinx Search (http://sphinxsearch.com) at all? It doesn't seem like it gets many releases anymore...since they unpublished the source code, it's hard to see how much activity there is.
https://manticoresearch.com/ is the lively, open source, fork of Sphinxsearch. that's where some of the earlier developers from the project moved to. it's used as a text-search backend on craigslist.
I'm using a 2.x version of Sphinx and have been for many years. I refuse to upgrade the version. I haven't been able to break/crash the version I'm using under nearly any common circumstances or loads, so I'm sticking with it until I find an alternative that is dramatically better. I've learned every nuance of it over time and can make it sing & dance exactly how I want it to. I consider it a spectacular piece of software; it does a thing and does it well, reliably.
We just migrated from Sphinx to using the full text search indexes in PostgreSQL, we had to deal with some changes in how special characters are handled, but it's worked well enough.
As far as I remember few years ago they didn't have BM25 and even TF-IDF support. Have they added that? Are you experiencing any issues with full-text search quality after migrating from Sphinx (you probably used BM15(+F) which is BM25 w/o doc length).
Do you have any numbers on your requests or searches per second in a real use case? I've really been wondering this as I've been considering manticore which is the major sphinx fork.
I see you don't have autocomplete in the search box. You might be interested in this interactive course https://play.manticoresearch.com/simpleautocomplete/
It's about Manticore, but may be used with Sphinx too.