It's true that 128 is better to use than 120 but the main problem is not in the value but in the fact we never get
CurrentRunningXacts->subxid_overflow = suboverflowed; with value more than 120. This solves the problem but it doesn't seem the right way to fix the issue. Instead it's better to process suboverflowed state which is legit itself not resulting getting the crash on the Assert. So the discussion of "better" value doesn't seem related to the problem. It is for demonstration only.