What's the thing with the syntax? If you don't intend to use the type elsewhere don't give it a tag, if you want, you have to give it a name. (Assuming you are annoyed by the duplicate Tag)
If the user of Tag is supposed to know how the internal details:
struct Tag { ... };
if it needs to rely on the internals, but the user shouldn't care:
typedef struct { ... } Tag;
if it can be opaque (what I would default to):
typedef struct {} Tag;
I also think that is a good feature to separate specification from implementation, I like being forced to declare first what I want to implement. Funnily in your special case you wouldn't need the declaration. (But of course it's a bad idea to rely on this "feature")
What's the thing with the syntax? If you don't intend to use the type elsewhere don't give it a tag, if you want, you have to give it a name. (Assuming you are annoyed by the duplicate Tag)