Replacing Comcast's gateway with own modem/router reduced speeds to 20% of what I had

Hello all!

I am at a loss. I realized I've given up too much money renting Xfinity's gateway for the past 5 years, and it was time to buy my own modem and router. Ended up purchasing an Arris Surfboard SB8200 modem (after ensuring it was on their list of approved modems) as well as a TP-Link AC1750 router.

Using Xfinity's gateway, I was seeing between 140-150 Mbps of my 200 Mbps plan. That, I can deal with. However, I set up the modem through the Xfinity app, attached the router, and suddenly saw speeds between 20-30. Surely that's the router's fault, right? I flipped a bunch of settings to no avail, and returned that router for a new Netgear Nighthawk AC1750 thinking the hardware itself was the problem.

Same exact thing. So far, I have:

• Restarted my modem and router multiple times
• Called Xfinity support, who suggested I restart my modem (already done), suddenly said they would activate my modem on their end for me (which I thought I had already done through their app, like they told me), and mentioned that my normal speeds should be seen within 24-48 hours. Weird that I have to wait that long for speeds that can be immediately re-retrieved if I plug their gateway back in and reactivate it.
• Fiddled with the channels on both bands of the router
• Switched NAT filtering between Secured and Open
• Disabled/Enabled WMM
• Disabled/Enabled QoS
• Selected Dual Band and Auto channel selection for my Windows 10 PC's wireless adapter (which reports speeds of 866.7 Mbps on its Wi-Fi Status window from the Network Connections menu, for what it's worth)
• Flipped between connecting to the 2.4GHz and 5GHz channels, which both report the 30-ish Mbps speeds. Connecting directly through Ethernet reports the same.
• Yes, Xfinity, I've used YOUR speedtest site as well rather than just speedtest.net and rest assured, it still reports less than 30 Mbps.

I just have hit a wall and have no clue what to try next. SURELY Xfinity would not throttle your speeds on purpose just for trying to break out of their $10/mo proprietary equipment jail cell, right? ....Right? Somebody tell me I'm right.

Any suggestions, tips, or further advice would be greatly appreciated. Thanks for your time!