Is this the fourth prototype or do (did) you have product(s) for sale?
I don't think you need a larger battery since most individuals aren't going to use them for 25 mile rides.
Hydraulic brakes are a great idea and help separate you from the fat tire behemoths that aren't who you want to compete with. Don't let the individuals who ride 10 mph convince you to change.
Don't recognize the mid-drive, but if it isn't Bosch, Brose or Yamaha, you may have difficulty selling the bikes. I think you would be served better with a front hub like Gocycle.
It should be fun trying to get very many batteries in a narrow seatpost, and if you increase the size of the seatpost/frame, weight will increase. Before you consider using the frame tubes, you might try to find someone from Faraday Bikes (now defunct). They had a great system and I really enjoyed testing their bike, but batteries in a steel tube seemed to be a problem.
The Chinese can manufacture excellent products if they're specified correctly and someone is there to monitor quality. My granddaughter's Kent e-BMX is virtually perfect and priced very well (even before it was discounted).
Do what you can to keep the price at $1995 if this is feasible.