I think a tag wiki with just a bare URL should be rejected so you have my full support on the proposal outlined in your Question.
This may sometimes be coming about because new tag wiki writers are trying to write the tag wiki itself where the tag excerpt should go, because it is presented first. Having thus written the Tag Wiki in the excerpt box, all ...
As per the Research Assistant badge requirements, is it possible that you have only been editing tag wiki excerpts and not the body of those tag wikis?
I just checked a couple of your tag wiki edits and they look like they were excerpt only.
I thought this was just because you do not yet have the approve tag wiki edits privilege that comes in at a reputation of 5,000.
I also wondered whether there might be a tag wiki edit pending.
When I look at https://gis.stackexchange.com/tags/error-000666/info with moderator privileges I see:
so I could easily create the tag wiki by clicking Create Tag ...
When you click on the hexagonal-grid tag you should reach the page below:
click on the help us create it link and at your reputation you should be able to suggest edits to that tag's wiki and excerpt.
The Approve Tag Wiki Edits privilege only becomes available at a reputation of 5,000.
If you are unable to suggest the edit as I describe then please let us ...
I agree with you, but there is one exception:
When there is no tag excerpt and no tag wiki (i.e., it is the first edition), the Edit Summary box is not available (see screenshot at the bottom).
So, if an excerpt is suggested for the first time, and it follows at least part of the tag excerpt policy, the only place to write a reference is the tag wiki box.