Your battery idea is almost exactly what I am doing for my current build, I am using 20s too, to charge I have a cellman switch charger (21s turned down to 20s) with a 20s Signalab bms mounted on it which when finished will have 4 cell logs mounted on it too, each displaying 5 of the 20 parallel string voltages, this connects to the bike with a d-sub25, 3 pins grouped together for - 3 pins grouped for + and 19 for intermediate cell taps. So I can do either 50% storage, 90% bulk or 100% balance charge. The cell logs will show me the cell voltages when I connect the charger, any out of balance needing investigating will show up and also confirm bms performance when charged. A plug and play 20s bulk/balance charger which also shows all the 20 individual cell string voltages.
I will rely on the CA for LVC but set it very safe at 73v.
I have not decided on which cells yet, 4s hard packs would be cheapest, 10s packs will simplify the wiring a lot! I will go for 15,16 or 20AH again not decided yet as I still have the main frame to design.