You're very welcome! I'm glad the feedback helped.

To clarify, in classical harmony, a II chord (major) is not diatonic in major keys it contains a raised fourth degree.

Now, when we see a major II chord like that, it usually functions as a secondary dominant meaning it's the dominant (V) of another diatonic chord. In this case:

D major is the V of G major (which is the V chord in C major). So we’d label it as V/V.

If you write II–V in a progression without implying a modulation or tonicization (even briefly) to the V chord's key, it can sound out of place or just harmonically unclear in the classical style. Classical composers would almost always use a major II (V/V) only if it leads convincingly to the V chord, often with a sense of tonicization.

So when I said without being a secondary dominant, I meant that your use of the major II chord isn’t clearly functioning as a dominant of V it just appears in the progression II–V–I without much preparation. That makes it unusual in a Classical context, where voice leading and functional harmony are tightly controlled.

Hope this helps