Present the most-used taxonomy terms to content editors first

The module Popular Tags does this, but i am torn on the interface. I think it is a good option if you really want to bias editors toward only choosing the top seven or so terms.

And you don’t have a million terms, as it has an option to show all of the terms (as links)

This is done under field settings not the content display field widget options, but you can just tell it what taxonomy term reference view to use to list the options to select from. Make sure you choose the correct vocabulary in the view! This is not enforced.

A good how-to: https://www.webwash.net/customize-entity-reference-autocomplete-results-using-views-in-drupal/

The problem is if using autocomplete, you have to type something to see some results.

While we could switch to TomSelect or such for turning a huge select list into a manageabel thing with JavaScript, that does not have the capabilities of placing the terms in a specific order selected specifically for that piece of content, which Tagify gives a good interface to.

This issue will allow entering into the field with a tab or a click to see suggested results, no need to type anything, and has a workaround:

Allow showing autocomplete results for Drupal fields with empty input string [#3081956] | Drupal.org