Ah site search, the never ending project.
Hot Tech Tip 1.
Pay for the Xenforo upgrade - use AWS elasticsearch for the end point. It is expensive but worth it (my setup is approx $60 US per month). I went this path for forum search because 50% of threads are invisible to Google and Bing* and the older community members who remember a thread from way back and want to dig it up really appreciate it. Plus, as a separate AWS instance dedicated to just search it's super fast and puts no load on the rest of the forum - most importantly though, the results are relevant and on point.
Hot Tech Tip 2.
Bing Custom Search service on Azure. If all your content is public then nothing comes close to Bing custom search (yes, it *is* better than Google's site search offering) - plus will be less than $10US per month. Easy setup, walk away nothing more to do. | Example.
* The whole of the Apre section is excluded to Bing & Google bots to reduce signal-to-noise ratio for overall site SEO.