I had a similar issuewith encfs encrypted mounted folders that I couldn’t get to be searched by overview search no matter what.
I like the overview search and all these fancy search providers, but in the end I decided that a search that does not reliably finds my stuff is of no good use. Currently, I only use search for starting and switching to apps.
For searching files I use fsearch.
That’s not an ideal solution, but at least it works.