Thanks to @idle @Dakspyker for pointing me at the Tag Wrangler plugin for #obsidian
https://github.com/pjeby/tag-wrangler
So all this time I had the tag pane disabled (!) and had totally missed the “list of all tags by frequency of use” list, which has prompted a certain amount of cleanup and tidying to at the minimum try to fix single-use tags. Tag Wrangler helps that effort.
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Yes, I really think periodic review is important for getting the most out of a notes collection. Tag review is a great way to do this. Looking for ways to recategorize notes or merge tags under a hierarchy means reviewing.