I've got similar issues:
SB6183 installed 2/17/2020 100 Mbps sservice
Initial downstream power levels -7.1 to -7.5, upstream levels 47.1 to 47.3, error log empty
Couple weeks later downstream levels -8.1 to -8.5 with corrected errors multiple channels, upstream levels 49.5 to 50.1 , ranging errors logged infrequently
Week later downstream levels -9.1 to -10.0 with numerous uncorrected errorson multiple channels, upstream levels 50.1 to 52.0, errors numerous, random modem reboots
Up one morning with modem repeatedly rebooting. Did factory reset and connected with downstream --8.1 to 8.5, upstream 48.5 to 49.5. Occasional errors logged. Signals degraded over next few days and when upstream values at 52.0+ modem continuously reboots. Repeated factory resets a couple more times over next two weeks and contacted tech support. Support analyst knew just enough to be dangerous (and wrong) but keyed me into the Activate option to reinstall my old (unsupportrd because it does not allow Comcast remote access, but works when registered as it is DOCSIS 3.0) SB6121
Fourth instance of repeated rebooting so tried SB6121. Got it installed but could not connect because COMCAST had removed its MAC address from my account. Got the weekend tech to register MAC over phone and all was well. Awoke Monday Morning to no Internet. Talked to tech and they had lost the MAC address. Could not enter it again because it is an "UNSUPPORTED MODEM". (I noticed in the SB6121 that Comcast had downloaded a SB6183 configuration file that was aborted but the modem was running just fine up until monday morning. Repeated factory reset on SB6183 and reinstalled it. Am watching the signal levels get worse by the hour and the error log fill up. I am convinced I need a tech out to check the line levels to the house but they are playing the COVID-19 card.
Your upstream levels are not quite as bad as mine, but I suspect you need your line checked also. Good luck in getting anybody who knows anything to show up.