Multi cell lipo charging or boost converter
Sounds like a high-discharge rate application where quick battery  replacement would be ideal.  Have you looked into power tool batteries?  All the safety and cell balancing is built into the packs and the pin outs are well documented on forums.

If you are looking to minimize cost, weight, and maximize efficiency stacking cells wins hands down every time. The only thing paralleling cells does is eliminate the cell balancing... which at that voltage can be done with a single IC.