BodoM

Here are my thoughts, after further research, including links to key words used:
https://www.google.com/search?q=pylontech+prolong+lifespan+cycle+soc https://www.google.com/search?q=extend+lithium-ion+battery+lifetime+pylontech  
Results from further research is noted beneath each referenced URL.
One should "Charge them up to 100% regularly (at least once a week) and keep them at that level for an hour or two to do some balancing."     I've not managed to find ANY other reference site that says this is best practice however!   Potentially the Pylontechs even do some "own balancing" when above 90% SOC, I'll write to Pylontech and hope for a response on the "regularly charge to 100% to balance" recommendation.  
https://batteryuniversity.com/article/bu-808-how-to-prolong-lithium-based-batteries
The LOWER the DOD is, the MORE cycles you'll get out of your batteries.  (Table 1) "Cycling in mid-state-of-charge would have best longevity."   --> in other words, pick your own SOC levels and swop between SUB (bottom end) and SBU (top end) and keep cycling! Cycling between 75% and 65% prolongs cycle count dramatically  (Figure 3  https://batteryuniversity.com/img/content/DST-cycles-web2.jpg)  
https://www.youtube.com/watch?v=Yf9N9zBgyB8
It is NOT advisable to charge the battery to 100%, as it degrades battery life  --> https://www.youtube.com/watch?v=Yf9N9zBgyB8&t=225s Best SOC cycle range is close to 50% --> https://www.youtube.com/watch?v=Yf9N9zBgyB8&t=318s  
https://www.4x4community.co.za/forum/showthread.php/331255-Lithium-Battery-Life-Expectancy?p=4415198#post4415198
"Ideally, your li battery should be charged to 90% max and discharged to 80% DoD (20% SOC) to prolong it's lifespan.  That's the "perfect " charge / discharge limits..(as per battery university)"   -->  could not find a confirmation on the batteryuniversity.com site to confirm the 20% SOC "low" level, but the 90% "high" level seems in order   
https://www.electronicdesign.com/markets/mobile/article/21190344/proper-care-extends-liion-battery-life and https://news.umich.edu/tips-for-extending-the-lifetime-of-lithium-ion-batteries/
"Using only 20% or 30% of the battery capacity before recharging will extend cycle life considerably." "Avoid charging to 100% capacity"  
My setting that I'm using now on the "UPS" will therefore be to cycle between 65% SOC and 85% SOC, until such time as I've heard from Pylontech whether it is necessary/advisable to bring the SOC up to 100% at all. 
I'd welcome any further thoughts or critique.