WernerM

Dip switches are all down in the same position for all batteries. 
Your batteries should be balanced before connecting them to the inverter. 
I did it slightly different. 
Step 1 charge existing battery bank to 100% SOC. 
Step 2 disconnect existing battery bank from inverter. 
Step 3 connect new single US3000C battery to inverter and charge it to 100% SOC. It comes shipped at 50% SOC. 
Step 4 disconnect new battery from inverter after 100% SOC is reached. 
Step 5 connect new battery to existimg battery bank and wait for them to balance each other. All led lights should go off and battery should go into standby when this is achieved. Mine took less than 20 minutes as they all were at 100% SOC. 
Step 6 connect the full battery bank to the inverter making sure that that the new battery is the master.

Your rate of "degradation" is exactly in line with my expectations.  Let me explain:
It is extremely difficult to estimate remaining capacity in a battery.  In practice it can be measured in a laboratory environment by discharging at some set rate and coulomb-counting the output until it reaches a predetermined voltage.  So, what Pylontech has done is a fudge:  They decrease the reported capacity by 0.1% for every 10 full cycles of discharge. (Note that both the 0.1% and the cycle are based on the original capacity)
I have 8 US3000Bs that are extremely pampered - never charged or discharged at more than 0.2C, never below 20C or above 30C, never discharged below 30% SOC, never kept at 100% SOC.   Despite this each battery reduces it's reported remaining capacity by 0.1% exactly as it completes 10 more cycles. 
So, it appears that this 0.1% per 10 cycles is the baseline.
What I do not know is:
Will the reported remaining capacity be reduced faster if the battery is abused? Will the rate of capacity decrease become lower as the battery ages?  If you look at the curve from the US2000 performance data (supplied by Pylontech) the curve flattens out a lot with age.