Your design reminds me very much of the Dyno Roadster (gone but not forgotten) and its many clones.
Seems to me that you could use a frame like that, if you made a subframe to carry the seat a little farther back. (Same idea as a banana seat with sissy bar, but smaller and stronger.) I don't think you'd have to place your seat any more rearward than a traditional pillion position.
Building your own from scratch has its own virtues, and I have built many a bike frame and component myself-- but I hesitate to make anything from raw materials if I can buy something else cheaply and adapt it to do what I want. Seems like the going rate online for a Dyno Roadster clone frame is $180, or about $300 for a basic complete bike. That's cheap.
For many years, I rode a chopper based on a shorter Dyno Glide Deluxe frame like this one,
retrofitted with a 25" long fork, 18" ape hangers, and 20" wheels. I'm 6'8" tall and 350 pounds with a relatively long-legged build, and the only parts I had to make were a very long, strong seatpost from 8ga chromoly tubing, and a special steer tube to convert the triple-tree style fork to a stacked crown conventional fork.
I recently converted that frame back over to 26" wheels and a normal length fork to make a power-assist bike for my wife, but I can still ride it with no fit problems. I used a BMX handlebar to allow a spacious cockpit that works adequately for both my 5'8" wife and myself (though the bars must be tilted differently for best fit).