Gigabit service running at 600Mbps... Multiple calls No Resolution

I am having the EXACT same issue as this user's post here - Gigabit service running at 600Mbps... Multiple calls (details) @somegeek 

• I was offered Gigabit plan late 2019. I took it.
• Ordered new modem from Comcast, the CGM4140COM, replaced old modem.
• On Google Wifi, speed tested directly from source on cat5 cable. Initially got 950mbps for first three months.
• In Jan 2020, speed dropped to 600mbps and remained here ever since.
• I've had 3 conversations conversations with Comcast Support with varying degrees of expertise from absolutely junior (no idea just reading a script) to moderately experienced (knew about the issue, and suggested to replace my less than 4 month old modem).
• Scheduled technicians twice. Twice technicians verified my signal is balanced and clean. Very clean in fact. Both times technicians did not have equipment to test my speed.

Every interaction with Comcast lasted more than an hour. I spoke to Comcast 3 times with customer service over the phone / chat. 2 technicians stop by to tell me my signal is clean and balanced. Nothing is resolved.

I remembered several years ago Comcast technical support used to be able to run a speed test remotely directly to my modem. Why can't they do this anymore? This is frustrating. I am being sold and paying for a service I am not receiving, which feels like I'm being cheated, swindled, and given the runaround.