Re: 2v batteries or 6v - which is better?
One analogy might be--you need ten gallons of milk.
You can buy 2x 5 gallon containers or 40x 1 quart containers. You still have 10 gallons in the end.
But, perhaps your refrigerator cannot hold 2x 5 gallon pails... So you get quarts.
Perhaps you cannot lift 5 gallon pails (40+ lbs) to put the milk in the top of the soft serve machine... So you go with quarts.
But, now you need to hook this to a dispenser--You can drop two hoses in two pails or 40 hoses in 40 quart containers. Big pain to keep 40 dispenser hoses clean and running.
Nothing right or wrong with either solution--but you look at your needs and abilities.
Technically, paralleling lots of batteries is sort of difficult to do (current sharing, wiring, lots of connectors), each string needs a fuse+holder (another $30-$50 of hardware), etc...
Note that each device type has its own idiosyncrasies... Batteries being voltage devices do not parallel / share energy well. Solar panels being current source devices (more or less) actually share current very nicely.
For a 48 volt bank, taking care of 24 cells once a month for water and specific gravity checking is doable... Get 4 strings, and you are now up to 96 cells to check.
And for debugging, many times it is easier to find a bad cell in one series string (battery bank voltage drop 2-4 volts with a dead cell--just take a DMM and measure each battery/cell).
if you have 4 strings in parallel--you can lose "one cell/battery" and the not really see a drop in voltage/performance (a 25% performance hit will not be obvious in an otherwise "good" bank).
You can purchase a DC Current Clamp meter for $100 or so and check for unbalanced current output/acceptance for each string in the bank to look for problems. Are you going to do this once a week/month?
Again, this is my personal bias--I would recommend aiming at one string of large capacity batteries--and no more than two or three parallel connections.
And you have a choice of cells to meet your ability to transport and install... Big 24 volt (or even larger?) forklift/traction batteries (need fork/crane to move/position)... Or some nice 500-1000 AH 2 volt cells that can be moved by one or two people without a crane/lift.
You are correct that AGM/Sealed batteries do not cells to check water in--but they still need fuse+holder, proper wiring to balance current loads, and still have the issue where one or more "failed" cells can take a string out of service (if cell is open) or even discharge the rest of the bank (if internal short).
I have used current clamps, and even used a DVM set to millivolt scale and use identical cable sections in different strings as an impromptu current shunt (the cable in each string should have the same drop across the bank when under load/charge).
20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.