adding such a highly expected piece of functionality like deleting something you uploaded should probably be more highly prioritized.

Let's be clear that this problem has nothing to do with Lemmy or the Lemmy devs. This is not a problem of prioritisation and it is not something that can just be fixed.

This is a problem of ActivityPub and the nature of federated services. There is just no way you force a federated server to delete something. So while most servers are well behaved, others might not be. Such servers could be defederated but you need to detect such bad behaviour first.

The problem is not with Lemmy, it's the whole Fediverse and ultimately also the Internet that works like this.

(the problem of a user uploading a photo and never posting it anywhere and wanting that to be deleted should be fixable though)