Really need an expert for this speed issue

Ok, I have tried everything and can't solve this. I have had comcast come troubleshoot. No one can figure this out. I need an expert. Here's my situation.

My Equipment
I pay for 1.2gb speed. I have an Arris S33 modem. I have an Asus RT-AX86U router. 

The problem:
I can't get more than 950/40 speed. My speed range is usually 850-950 download with consistent 40 upload. (tested on speedtest.net and comcast site)
The first Comcast tech came to my home and I said I had a feeling it had something to do with how comcast provisioned my modem. He wiped my modem off my account and rebooted and then I did a speed test and I got 1420/41 speed. Then I did another speed test a minute or two later and I went back down to around 925 speed. 

Doing every troubleshooting thing I could possibly think of nothing changed. A few days later at around 10pm I did a speed test and I got 1311 speed (that's with my router hooked up). I got this speed on my Samsung S21 Ultra so this was going through the wifi at that speed. Shortly after, the speed went back down to around 850-950 and not ever going over 950. The next night the same thing happened, about the same time of night, I got around 1300 speed again. It's been a few weeks, and I have never gotten speeds over 950 again.

Comcast came again and sent a supervisor. They checked my lines, no problem. Replaced any cables that could possibly be an issue, no problems there. All tests show no problems on their end or with the lines. The supervisor checked the speed with his tester at the street level and was getting a little over 1300. He tested with his tester coming right out of my wall, got the same speed 1300 or so. He checked with his SB7 modem and his laptop and gets 950 consistently, not higher. I tried connnecting his SB7 directly to my computer and got the same speed 950.  No matter what I do I can not get more than 950 speed (Besides the 3 times in the last few weeks I got over 1300 speed).  No one can figure out why.

What I think:
To me, I have seen speeds over 1400 direct from my cable modem to my computer and over 1300 with my router over wifi. I am thinking there is some sort of artificial limit limiting the speed to 950 somehow. They said the modem config file is correct. If I am paying for 1200 speed and sometimes can get as much as 1400 speed, I would expect the speed tests to be all over the place. Sometimes 1200, sometimes 1075,  sometimes during peek times maybe dropping as low as 950 maybe. I'd expect at 3am, I'd see my highest speed tests. But this isn't happening. It seems like 950 is my limit and when there may be network traffic issues it goes down to 800-850. But the fact it never goes over 950 when I have seen it be 30-40% faster makes me believe this is some sort of issue. I know I can't expect to get 1200 speeds, but I do expect to get close, sometimes, to that speed.

What I Have Tried:
I swapped my S33 modem
I tried a more expensive router
I have Cat 8 cables all new
I tried going from my cable modem directly to my computer
I tried using a different computer in the house
I had comcast take off all of my equipment off my account and add it again
I have tried factory resets of modem and router
Lots more I can't remember ATM

Added info:
A year ago I had different modem and router and signed up for gigabit service (1000 not 1200). Comcast said "OK, everything is updated on our end. Reboot your modem and you should get the extra speed".  Before I was paying for 250/5 speed and always consistently got 250/5. When I did the reboot my speed didn't change at all. They blamed my equipment and said my equipment couldn't go faster. I knew it could. I told them that if my equipment could only go to 400 or 600, I would expect my speed test to go up to 360 or 550 or change in some way. I ended up going to comcast and tried their modem and sure enough I was getting 900 speed with their modem. I told them I wasn't ready to buy new equipment and I didn't want to rent a modem out, so I asked them to put my modem back on the account and I would return their modem. And sure enough when she did that, my speed test on my old equipment (that they said couldn't go faster) gave me 600 speed. About 5 minutes later it went back down to 250 speed (what I was paying for since they took me off the gigabit plan). So my current problem feels the same way. Everything on their end says I should be getting the faster speed, but there seems to be some artificial limit on their backend somehow that I can't explain.

This guy seems to have a similar problem

Speed bump from 1Gbps to 1.2Gbps not working | Xfinity Community Forum

So any help with this would be appreciated. I have wasted many hours trying to figure this out.