Oh my goodness... this dress is so up my alley. I can't wait to see it coming together!
For the peplum belt, I like the idea of doing it with black fabric, and using the tweed as the sash/middle, as that would give it even more drama, but it already has a lot going on, so if you wanted to keep the middle less dramatic, and keep the eye on the neck/sleeves, then obviously the tweed with black sash would be a better option.

I used to sit for hours and sketch out fashion designs, but I've never actually made any of them, just lived vicariously through my drawings. haha It must be such a feeling of accomplishment to be able to make something of your own design. And so much fun to be able to customize everything!