No we have not a clear policy on only linking sitelinks to categories if the item itself is about a category. So not let's not break that.
You suggest to break down almost the complete navigational structure Commons has in relationship with Wikipedia, and makes it possible to find articles that are about the same subject as the category. Without it becomes almost impossible to identify a category on Commons to be related to an article in Wikipedia.
Sorry, but your proposal is insane and making the navigational situation a thousand times worse. And does it make anything better? No, totally not. Only the opposite: worse.
Wikidata is currently heavily used to connect categories on Commons to articles on Wikipedia. This so that interwikilinks are shown on the category on Commons to the related Wikipedia article. This for navigational purposes but also to uniquely identify categories on Commons to articles on Wikipedia and items on Wikidata.
How nice Commons galleries are giving an overview, they are crap in speaking of navigational purposes. For every subject a category on Commons is created and used and the Commons categories form the backbone to media categories.
It has been pointed out for a long time that the linking situation on Commons is problematic and this is a software issue, not a user side issue. This consists out of:
* There can only be added one sitelink to an item.
* If no sitelink added (but only added as property), a Commons category can't show the interwikilinks.
* If a category and an article on Wikipedia/etc exist for a subject, only one of them can be shown on the Commons category.
The annoying part is that some large wikis, especially the English Wikipedia, creates too many categories that are not created on other Wikipedias. This causes that categories on Commons are only linked to a category on Wikipedia, which is useless for most other wikis and on Commons we miss an interwikilink to the related article.
A gallery on Commons is a great way as alternative to show images, but is not suitable for navigational purposes, as that requires a much higher coverage and being a backbone everything relies on. On Commons only categories have that function. A counter proposal makes more sense: no Commons galleries as sitelinks any more and having Commons galleries only as property added.
But this only solves a part of the problem: on Commons I would like to see somehow that both the related category as the related article are shown. Example: on the Commons category for a specific country both the country category on Wikipedia is linked as the article on Wikipedia is linked.
Something I have been wondering about for a long time is why there are 2 places on an item where a Commonscat is added. I understand the development and technical behind it, but this should not be needed.
So the developers of Wikidata should try to find a way to show both groups of interwikilinks on categories on Commons.
As long as this is not resolved in software, this problem of 2 items both strongly related to a Commons category keeps an issue.
Romaine