These problems aren't unique to tags, they're already present in subreddits names. For the most part it doesn't seem to be a big deal. As long as you can see the number of subscribers to a certain tag when creating your post, stackexchange style, then you'll probably end up with the tags you intended most of the time.

Exactly. You just need to take a look at www.reddit.com/r/subredditsashastags

I'd like to see some 'AI' assisted tagging, where users are prompted to use a tag from a pool of common tags with the AI making suggestions on what might be appropriate, this would address most of the issues listed above.

Pocket does that with the premium subscription. I find it to be quite useful.

Something like Stack Exchange's tag system would be enough.

I'm not massively familiar with Stack Exchanges tag system, it does have some synonym handling, https://blender.stackexchange.com/tags/synonyms , that seems limited to one alternative though, is there a more advanced aspect that I am missing, perhaps covering sememes?

Can sort by Target to see the (sometimes) multiple synonyms for a particular tag.

Tags can be curated or moderated like anything else, that's just an implementation detail. Sites like Twitter and lobste.rs seem to get by just fine with them..

how does stack exchange do it? they have aliases of tags that merge into the authority tag

thank you for this. having fought against tags before for the same reasons i sometimes think the world has gone mad...