The lithium capability of operating happily without a full state of charge would be a marvelous thing. I can hardly imagine it as my own PV/battery system was designed for lead acid's need for being fully charged, and when I think about batteries, I immediately think of how to keep them topped off to extend life!
For wet lead-acid, the true deep cycle batteries are a horror show of inefficiency. For good life they must be fully charged, but charge efficiency for the last 10% is near or below 50%, and they eat power like crazy even when full. The self discharge rate is terrible. The floor polisher battery (6V) I got recently for my welder sucks 2 amps continuously when topped off, and this is within the manufacturer's specs. I was shocked.
A much better lead acid technology is the AGM batteries, which don't continue to eat power when full, and have high charge efficiency. They are what I designed for. The cheap semi-deep cycle, lead-calcium batteries I'm using don't have performance as good as AGM, but vastly better than wet deep cycle lead acid in that the charging cycle current does taper to nearly nothing (50ma). Lightly loaded and charged at moderate currents, the Lead-calcium batteries are a decent value. By using 120V instead of 12V, keeping actual battery loads to a few amps, and DOD down to 15% on average it's a different situation. My batteries are fully charged every day long before noon, but I don't have to wait for that, I have enough excess PV power during the morning for big loads. In fall, my batteries are nearly full but not in float by 10AM. Direct DC use and propane refrigerator/freezer is a big plus, there is zero load at night when I turn off my lights and computer. The inverter is only on when needed, though it only draws 15W when idling. A house heating cycle draws 20 Watts for about 4 hours from my 12V AGM battery for the circ pump, daily in near zero F weather, every other day for most of the winter due to thermal mass and super insulation. Yes, I am a fiend about low power design.
If/when lithium ever gets down in price to compete with my lead-calcium batteries for ongoing replacement cost, I'll have quite a project to design a new BMS for a 120V series string of cells. It's good to learn that I can do cell balancing slowly at night, as I'd need nearly 40 cell regulators instead of your 4.
Thanks for sharing your pioneering project on lithium batteries!