It seems to be my problem is sorted, but they have nothing to do with that, in fact they have no idea how CAN bus comm works...
Solution: Replaced my Sungrow inverter as it was not quite the right size and capacity for my needs. New inverter is a Deye Sun 8k unit. As soon I wired up and connected the two units, communication were established and seamlessly running in the past two weeks.
Conclusion: In order to figure out what is/could be the difference, the only thing I could point out is the missing ground (CAN ground) on the Sungrow unit. There were only CAN-H and CAN-L terminal, and I presume (no documentation regarding this at Sungrow, and they were unable to answer my question via phone) there is a common ground connection for the CAN and the battery system which is not marked and obviously I did not discovered until now. The lack of reference, the communication could jam during certain conditions which even leads to the "lockup" of the entire BMS.
During the fault finding period (zoom meeting) with Batrium, I sent pictures of both end of the comm line, and for my question whether the ground is important or not , due to the missing terminal on the inverter, the answer was a definite "... this thing has nothing to do with any of the issues...".
To be fair, I'm still not 100% convinced, as I had experienced "lockups" when the CAN was disabled back then... so I'm nervously calling the issue resolved.