Did you check the efficiency / operation of the catalytic converters before you replaced the 02 sensors.
A) looking at live data on a scan tool, you should see the 02 voltage on B1S2 and B2S2 remain steady high at around 0.7mv.
B) you can use a infrared thermometer to measure the temp at the entry and exit of the cats. The exit temp should be around 30-50 degrees hotter
C) you can measure back pressure of the cat by removing each pre-cat O2 sensor and using a pressure gauge. Backpressure readings at idle should generally be less than 1.5 psi (10 kPa)
Do you still have the old rear O2 sensors? If so you can buy something like eBay item 282256410019. This has a mini cat in it and is used as spacer which eliminates check engine light for catalytic inefficiency DTC's
You should also check the wiring harness to make sure everything is in tact without any breaks or shorts.
My car is also has a LLT engine and I did an ECU software update which increases the threshold for the catalytic converters to determine their effeciency. This update was from GM tis2web
https://tis2web.service.gm.com/tis2web/