It seems to Vary Greatly with batteries.
Got 2 batteries in a Large Diesel truck.. 11 years old and doing FINE... maintained with only distilled water for top off ..
and then have 2 batteries in a Dump Trailer, DEEP cycle.. which will not take a charge after only 3 short years.. also maintained by trickle charging.. Just replaced 4 batteries in a boat ,cuz we cant take a chance with those.
Once the plates Corrode, imo they cant be "reconditioned".. i highly doubt any of those Epsom Salt videos work. and if it did , it would be very short term.
Since you have this many batteries at stake, I would sacrifice one and cut it open and check the condition of the Plates.. that should tell you the whole story. Remove the Acid solution first and neutralize it with Baking Soda and water.
edit,
but then anything is possible i guess.. .I just recently learned that Plain Vinegar and water completely dissolves RUST.. I never knew that before, so simple
.. it works really well too on tools, anything.. So there may be a way to save those plates also .. I guess you can always experiment with the plates your remove from the one battery, if you got the time... not sure what chemicals or solutions would do that however..