I just discovered, that the 'Labels'-field has been moved to Module Settings (Module Settings > Basic Settings, below Module Title). And I now find it behaves mostly as we would like to achieve.
- It shows all the terms from the Vocabularies (but multiple: if you have multiple Vocabularies, it shows the terms of all of them; not sure if this is the right behaviour)
- It is a drop down, and you can only select from the drop down, not add by typing
- It gets indexed by DNN Search: so if you search for the tag, pages with that tag will show.
The downside remains, that IF an editor adds terms using the Tags-field in Page Settings, it will get added to this list. So management of this list would still be an issue.
btw: i still agree with the description in the issue, https://github.com/dnnsoftware/Dnn.Platform/issues/2883
But for the near future, a step toward the solution for our use case is a bit closer.
Just wanted to share this insight.