In the Drone World DJI flight times are already..Amazing most of my other Drones Battery times are between 3-12 min. (most around 5 min)
Welcome to the Forum from Bakersfield!
One issue with DJI batteries is they are mostly built with the intent to not be easily dismantled. Taking them apart is somewhat difficult and probably will be the same to put them back together. Post pictures as you progress.
Welcome and good luck with that. If you do manage to produce a working unit that isn't insanely priced: there won't be a shortage of people to buy it.... same with the Mavic 2 and Air2/2S.