Speed Issue with XB7 Gateway (3 Weeks)

I have had the XB7 gateway since the COVID no data caps went away. I have had to change the modem out based on recommendation from tech support every 3 to 6 months.

Most recently, I upgraded from 600m down to 900m down. However, the actual download speed still averages 600m down (+/- 20%). After calling to ask about it, the most recent modem was determined that the 'boot file' on the modem was stuck at 600m and would not keep the 900m file after a reboot. Changed the modem out again, and the boot file finally stuck, but speed was still not coming down from modem to ethernet over 300m with the new modem. Tech support was testing it to the modem at being the full 900m.

They ended up sending out a tech (subcontractor) to test the modem and line from my outlet to the street. The line is capable of handling 1.3 Gbps confirmed by the tech's tools, but the best the XB7 modem could send out via ethernet or 5Ghz wifi was 600m. He also said that none of the modems are able to go over 600m even when the path back to the Central office is capable of 1.3Gbps.

Funny enough, the path from the comcast modem to my netgear router to a computer was producing better ethernet speeds than the direct connect to the comcast modem (300m direct to comcast modem vs 600m through my router). 

Can anyone point out why the comcast modem that is connected to the coax line that is capable of 1.3Gbps cannot reach 900m (+/- 20%) at all even during low traffic times? Is the 'premier' modem really just that terrible? It seems like I am almost better off eating the extra $5 for the unlimited data and using my own modem because the XB7 goes out constantly and cannot reach over 600m.

Edit: Well since the Xfinity agents here are avoiding the question, I guess I'll go through here: https://support.xfinity.com/svp-contact-form