Maybe something like this:
Look what I found on AliExpress
DC DC 8~32V to 45~390V Adjustable High Voltage Boost Converter ZVS Step up Booster Module Capacitor Charge Board New Arrival-in Integrated Circuits from Electronic Components & Supplies on Aliexpress.com | Alibaba Group
(This one is only 200mA output)
To directly feed the pack.
With a half full pack, you should not have any balancing issues. And you can assemble your cells to low voltage configurations.