Jerome, you are correct about the batteries, series batteries will provide more power than parallel batteries, again assuming everything is up to it.

Essentially series/parallel works in opposite directions, depending whether you are considering a power source (batts), or a load (motor).

Of course this is all just theory, I should point out that I don’t know didly about actual starter boxes, not sure if this is what you should do. Gearing needs to be considered when jumping around in configurations like this, just like we do with our cars gearing.

Hope this helps more than it confuses. Cheers!