The usual scenario for telecom site batteries is design for long float life with perhaps a dozen or so major discharges. But long term survival at low voltages isn't in their job description.
Paying up the yangying for a lifeline means getting incredibly thick plates. Controlled overcharging sheds substantial amounts of plate material. But if they're super thick it's like having a checking account overdraft protection plan.
But the chances of survival for a battery with lesser thickness plates is less.float maintenance for 10 years will consume a chunk of plate layer as well.
By the way Don if you ever get your hands on a regular transformer rectifier battery charger you can throttle it with your variac. A Fred Flintstone grade sort of adjustable charger.