Packet loss, high latency, low speeds every evening

Apologies for the long post, but I'm trying to be detailed.  I've been having this issue for a few months now, progressively getting worse.  Everyday from about 6pm to 11pm (mountain time) I start getting packet loss, high latency and slower speeds.  Once the problem 'starts' (around 6pm-ish, varies everyday) it seems to ramp up until around 8pm where it peaks and is really bad for 2-3 hours then tapers off over the next hour or two.  This happens for all devices before and after a modem restart during problem hours.  

I pay for 1200Mpbs which I get just fine (and sometimes even faster) during the first half of the day.  When the issue starts it slowly degrades all the way down to about 200Mbps on average (at this point my latency in games, for example, have at least doubled).  On 9-27 it actually got down to just 5Mpbs.  This is when I reached out and talked with live chat support.  They confirmed that they were also seeing about 5Mbit speeds at that time and scheduled a tech.  He came out about 4pm (9-28) before the issues had started and checked stuff (inside and outside).  Replaced something behind a wall plate and replaced the modem with a new model.  A couple hours after he left the problem persisted right on time.

On 9-29, to rule out issues on my end, I disabled all of my wifi networks and uplugged all ethernet cables.  The speed remained slow when tested via the Xfinity app with zero devices on the network.  I added just a laptop over ethernet and confirmed the usual issues were still occuring.  I also moved the modem to the media panel and connected directly to the service-in coax to rule out issues with my house's coax runs.  No change.

On 10-2 during problem hours, I drove a couple blocks over and connected to two different xfinity public hotspots and got the same results as I do at home.  I don't think the issue is just with me. 

I have documented my speeds as well daily graphs showing my ping times (to google.com) and packet loss for the last 2 weeks.  I've also started logs with PingPlotter and have been pointing it at 4 places: google.com, 162.159.134.233 (a Discord IP), one of path of exile's (game) IPs and also an IP of it's login servers. I believe these are good targets that do not deprioritize ICMP packets.

Here are some daily graphs from 9-21 to 10-1. Each graph represents a 24-hour day, midnight to midnight. This tool pings google.com 4 times every 5 mins and logs the average response time and any timeouts.  Note how everything is fine for the first half of the day then gets bad for the second half, consistently.
https://imgur.com/a/UWuXnIy

Here are some PingPlotter screens from 10-1:
https://imgur.com/a/Ds9ZrXg

Here is two screenshots of downloading the same file during and after the problem timeframe:
https://imgur.com/a/Ntu25gZ

Here are screenshots of the modem signals during bad times (various days):
https://imgur.com/a/DtZhp1S

Here are some screenshots of the Xfinity speed tests during problem times:
https://imgur.com/a/p2Od3Av

Notes:
-Fridays are usually the smoothest days.  Sundays are usually the worst
-There are no splitters being used as well as no other service using coax here.
-Peak problem time is around 8-9pm mountain time.  Problem windows is around 6pm - 11pm
-I've had Xfinity here for years up to this point without issues.
-Speed tests are done with the Xfinity app
-Problem does not improve when all devices (ethernet & wifi) are removed from the network
-All cables/jacks/wall plates/etc have been checked by two different Xfinity techs

I'm open to trying anything else that might help.  Thanks.