Consistent, repeated slow downs to 30mbps on 100mbps / 200mbps service?

100 mbps service, new modem (MB8600) in March. Everything fine during online school / WFH through mid-June, and I recall there was an outage / some area work around that time. Since then seeing consistent sustained speed drops for past ~5 weeks with DL of only ~30mbps, but upload speeds are unaffected (~6mpbs). The ONLY thing that works, is power cycling or resetting the modem, but this only lasts for a few hours to a couple days. Restarting the router has no effect. It "feels" like plain old throttling somewhere.

Initial setup / info

• MB8600 modem
• Google Wifi router (not the Nest)
• Internet only service, cable comes in the wall from outside directly to the modem. No splitters.
• Approximately 30 devices connected to wifi - phones, tablets, laptops, echo dots, smart plugs, TVs, etc. No new devices since issues started. 
• No indication from the routers or Xfinity data usage chart that anything is actually using the "missing" 70mbps (that would be ~700GB/day)
• Neighbor fed off the same box is also having issues, but techs I have talked to insist there are no "area issues" that could be the cause. His service dropped entirely one of the times they were working on my line, even though it was not physically disconnected.
• I have a raspberry pi hardwired to the router that captures modem logs and hourly speedtests for troubleshooting. Does not appear to be weather (hot/cold, dry/humid/rainy) or time-of-day related. Here is a chart of the troubles:  https://i.imgur.com/0XGKBXJ.png . Speed tests from both routers are within 10% of the raspberrypi results (though the pi has a hard time keeping up at 200mbps).
• Modem logs show many warnings/notices for Channel 159 but SNR/dB/errors do not appear abnormal

Things that have been tried

• New modem (SB8200) - same behavior. Power cycling restores speed.
• New router (eero) - same behavior, rebooting router does not resolve issue
• Techs replaced part of, and then all of, the line from the outdoor box to my house - same behavior, temporarily resolves issue since modem is reset/power cycled
• Multiple techs suggested that my issue was having too many devices connected, so I upgraded to 200mbps plan - dropped down to 30 mbps within a few hours of upgrading. 

Only thing Xfinity techs have left to do is place an Xfinity modem/router, which is happening Saturday, though I suspect that can not overcome whatever is causing this.

Has anyone else experienced this behavior, and if so, what was the resolution?