About the cycle life it depends on the battery and the manufacturer. What cycle life was advertised for your battery at 80% DOD?
Also the runtime your UPS can depend on the type of circuitry and it’s efficiency. I’ve used real crappy ones with really low Power Factor and they lasted only about 2 minutes even though my battery was capable of providing power for more than 20-30 minutes or so.
It’s a good choice but then he would have to use a balancer circuit for those.