Hi Juan,
Thanks for this interesting post. Wish I had seen it earlier. First of all, you have the right ideas and you have made it into something quite nice. There are, however, inconsistencies in certain motifs that you have applied. For example, the opening 6 bar phrase follows a do-re-mi pattern (that is, the upper voice by convention should rise from the 1st degree to the 3rd). In your example, the entire motif is over 6 bars with an irregular rhythm, and the upper voice does not commence on the tonic but instead the dominant, falling to the mediant. The emphasis on the mediant at the beginning is unconventional for the reason that that the emphasis should be place towards the end of the pattern.
To help illustrate, I have adapted your opening idea and it should be apparent how such phrases might appear in classical works. After the do-re-mi pattern, I developed some of the ideas that leads to a perfect cadence, after which your shift to second subject might occur.
One final point I would make is that you should refrain from changing the meter immediately after the first subject. I would be interested to hear your underlying reasoning for this as is quite unusual, that is within the confines of common practice.
Hope this helps.