There’s a very good KM macro I found somewhere a while back that does a great job of keeping a central list of tags and autocompleting. Will try to find where I got it, but probably the KM forums.
Nope, in the also-excellent Zettelkasten.de forum: Quick insertion of tags with type-ahead search (using Keyboard Maestro) — Zettelkasten Forum
It isn’t exactly the use-case you describe, but maybe could be adapted. PopClip would be great at tagging, but it may be more difficult to use it to store a consistent set of tags.