Since some of the tags have very little traffic in them, and hardly anyone has even the +5 reputation, and seeing how the moderators are already busy enough converting old answers to comments, is it possible to allow 10k (or some other class of users) to suggests synonyms regardless to the tag reputation?
The best thing to do in this case is encourage your diamond moderators to periodically peruse the synonyms page ...
... and manually approve any that they know to be correct.
A bad synonym can be fairly dangerous, so we want to be a bit conservative here. You can also lobby for any synonyms you particularly wish to see implemented here on meta, of course -- and over time there will be more users with more upvotes in specific tags.