Is there any way to increase indexing frequency of macOS via or for Spotlight on macOS 26?

This is a good question I’m not really sure about, but why do you want this to happen in the first place? I don’t really experience problems in this regard.

If you are having issues it could just be that your index is broken. You could trigger a reset by adding “Macintosh HD” to the privacy setting that excludes folders from Spotlight, then waiting a bit and removing it.