As far as I recall Spotlight has been in decline for a while. I stopped using it round about the time, years ago I think, when it started including sources I didn’t want, like the internet, as top results, with no way to turn that off.
Recently I tried Spotlight on some other pre-27 (maybe even pre-26 too) Macs in the family and found its results disappointing, and these undesired sources listed still at the top.
IIRC, Used to be, one could select the main storage volume and not index it, wait a bit and deselect it; is that what you mean by ‘rebuilt’? Similar feature appears to be buried at the far bottom of the Spotlight Settings panel, called “Search Privacy”.
Now the choices are more fine-grained. I did experiment with turning off everything and turning off the keyboard shortcut for Spotlight. Initially, searching in Mail worked but some days later it was not so good and I also tried searching in a Finder window when I couldn’t find a file with LaunchBar, my preferred search app. No results. Hm. So then I turned on Spotlight for Files and its “i” items, and Finder searching worked again.
So I can suggest, if you haven’t already done this, to put your main volume in the privacy section, turn off everything you can in Spotlight Settings, wait a while, maybe even reboot, take volume out of Privacy, then one at a time turn on search options, and see if that improves results.
Haven’t changed to 27 yet so can’t answer that, but I think I remember reading that Spotlight has been fully redone for 27 (in technical ways I don’t understand), including reaching deeply into many apps so the “AI” can “help” you. After updating I need to dig into its settings to see if I can configure it the minimal way I want and see if it is as good as LaunchBar.
Looking forward to replies from 27’ers.