The bag plan doesn't appeal to me. Neat freak I guess. 52v sounds like a good plan, but I already learned the lesson in having a battery so much bigger than I would ever use in a single session. It's extra weight, and expense, I can't justify.
I actually like the bag idea not for 52v ( I found a bagged 36v 20ah for around $300) but for having a long range 2 battery set up. Mounting a removable rail currently used on my bike to the rear rack appeals to me.
As usual I need to honestly access if I would ever need that kind of capacity but do see the advantages if I went AWD with B.O.B. trailer. Then it all makes sense.