I know I'm joining this thread late, but please concider getting an electric spot welder or among one using a triac that you can program and time with a Arduino for the milliseconds needed to weld nickel strip, or nickel plated steal strips for building packs.I assembled these 6s 18650 packs for my large hex, run 6s4p and get 30 mins flight although it pushes the draw of the pack. Been considering exactly what you have said for the mavic, any idea or max amp draw on the mavic as these cells are rated for 2c although less is always better
Soldering damages and weakens the chemistry of the chemicals in the cells, and even worse weakens the vent seal on the positive side and can cause the lacks to catastrophically fail a a higher chance of happening.
Soldering any li-ion or any batteries is never recommended.
Be safe,
Scott