Awesome! I'm about to convert my Marin Pine Mountain custom build, which sits about like your Soma, only with a rigid steel height-corrected fork instead of a suspension fork. I also use that same Serfas saddle from your parts pics above, which works well for me since I'm pretty upright on my bike. And like you, I'm also considering the BBS02 750W mid-drive motor, with pedal assist, likely no throttle.