I've spent countless hours reading other posts on these forums, reading information on other sites, troubleshooting with Spectrum, troubleshooting on my own and I eventually hit a brick wall. So, I bit the bullet and purchased both a new modem and a new router to test with. Worst case, it didn't help and I can return them to Amazon quickly:
What do ya know... first speed test is in the target range and so are the next few after. So, I connect the Netgear R8000p to the new modem, just to make sure and the speed goes way back down again. I continued to switch the routers a few times to make sure I wasn't just getting a few lucky tests and the results stayed the same. I got my bandwidth with the TP-Link and I got 1/3 of that bandwidth with the Netgear.
I setup the Spectrum modem to test with the TP-Link and got the same results so I'll be able to return the Arris and get some of my money back at least. My Netgear R8000p is going back into the box after barely 4 months of use.
If anyone asks me for a router recommendation in the future, I will certainly not be recommending Netgear... what a disaster this whole situation has been. I did learn quite a bit, so at least there's that.