Isolating slow internet speeds?

I've read quite a few troubleshooting posts and even tried to work with online chat support a week ago; I hate when the sales pitch comes at the end of an unresolved issue.  I recently, 6/17, upgraded the speed of our internet from 200 Mbps to the 600 Mbps plan.  Prior to that I had had the 200/10 plan for quite a while, and after my 12 year contract ended and I had to renew the 200/10 plan, I would routinely get speeds upwards of 250 down.  When I updated the 200/10 plan it settled right around 200 down.  With working from home and everyone being home all of the time, I increased the speed to 600 Mbps and have never seen anything close to that speed.  On the 5 g band from my router to my iphone XR I believe I have seen speeds up to about 380 down.  My wireless connection to my PS4 increased to about 150 from something in the 20s.  I have a AC3150 TP-Link router, and it has a speed test built in, which I assume tests the signal it is receiving, and I have seen it as high as 488 down.  I have an Arris SB6190 modem.  I contacted chat support and have done some troubleshooting.  I have rebooted the modem and router, unplugged the coax cable from the wall and modem, unplugged everything and plugged it back in, plugged an ethernet cable from the modem to a laptop (speedtest results as high as 175 Mbps maybe).  Today I went into the control panel in the closet and hooked the modem directly up to the coax line from the street and connected my laptop to the modem with a Cat5e cable and still only see speeds as high as 175, using various servers on speedtest.net and even the speed test at Xfinity. 

I see a bump in wireless speed, but not nearly as fast as I am paying for, the router is telling me it is getting a faster signal, still not what I am paying for, but the direct wired connection to the modem is way slower than what I am paying for??  I'm guessing it is something on Xfinity's side, but why would it suddendly occur when I upgraded our speed. There are no splitters or filters in our house, we only have internet, and it is only going to one coax port in the house.  Thanks.