Why is my Cox Gigabit Service CRAWLING???

I would sure appreciate some guidance here. I try to keep up but I can get over my head pretty quickly so please bear with me. My problem is that, for well over a year, I have been paying Cox for Gigabit internet and Complete Care, but my system performance never really got close to that. It has gotten progressively worse and now it is almost unusable from my wireless devices. The ethernet connected desktop is usable but only tests at about 250mbps. 

The symptoms are that pages can take 15 seconds to load and sometimes not at all and when streaming I get some minor buffering. The wife and I are retired and stream tv in the evenings but other than that it’s pretty much just checking email and Facebook. These problems occur on our Chromebooks, laptops, and iPads.

I use Cox only for internet. My modem is now an Arris SB8200. My Wi-Fi is Google Mesh with five pucks to cover our house which is under 1900 sq ft. My desktop is a Lenovo connected to the Google Mesh via ethernet. Speed test results (speedtest.net) are pretty consistent:

• The desktop reports between 250 – 275mbps down and 37 up … although it seems slower than that in practice.
• Chromebooks and laptops on the Wi-Fi report very strong signals but only speeds of 20 – 35 mbps down and 35 up. Here too, the performance seems even slower than reported.
• Using the Google Home app it reports the router is seeing ~850mbps but for some reason the devices it feeds are FAR lower...  

Last week I finally called Cox Complete Care. I had to run through the story with four different agents. Each time I had to start from the beginning and comply with their instructions as they worked through their obligatory scripts. A warm transfer sure would have been nice.  In the end they agreed to send out a tech. The tech was out the next day and did the following: 

• Said the coax running from the box outside of my house (not the one from the street) to the router was bad and he replaced it.
• He showed me on his handheld device that he was seeing about 950 mbps coming into the modem and nearly as much coming out.
• After rerunning the tests from speedtest.net there was no improvement. He just said that “maybe there was something wrong with my computer.” He then left.

Following his visit I read through several forums and decided to replace my Motorola MB 8600 with the Arris SB 8200. I had been seeing huge numbers of corrected and uncorrectable errors on a few channels and thought it was worth a shot. The Arris has provided the same results as the Motorola.  I’ve also noticed that the SB8200 front panel lights indicate that both up and down are blue, I believe indicating that the connection is not allowing connection to DOCSIS 3.1.  The event log was also showing a lot of Dynamic Range Window Violations but I performed a hard reset on the modem this morning so most were wiped away. I’ve attached a file containing the Connection Status which I just took. It too was wiped clean with the hard reset this morning but maybe something will give a clue.

Thanks in advance for any help you might have!!!