Alright, status update on this. In the interest of eliminating as many external variables as possible I connected to a public "Cox WiFi" hotspot hosted by one of my neighbors, and I was able to achieve stable 10Mbps uploads from my Plex server at home streaming to my phone. Results were similar for other workloads, meaning that there are no bottlenecks on my home network or the local Cox infrastructure. Since streaming is consistently slower on non-Cox networks, I'm thinking a bottleneck further up the chain and not directly related to my personal hardware or Cox internet service. While the issue still exists, I am going to consider this effectively solved as any solution would be well above my pay-grade.
About that hotspot thing. I asked a chat rep if my modem was broadcasting a wifi hotspot. He said no. Is there any way I can check if that's true or not? They keep charging me for extra data so my $109 plan comes out to around $200.
What @BrianM said. To verify, just pull up the Wifi settings on your phone or laptop and see if "Cox Wifi" comes up in the list of available nearby networks. If you see one with the same signal strength as your local wifi, you're probably broadcasting a hotspot.
Regarding the excessive data usage, I can tell you that streaming services, particularly 4K Netflix, add up very on your data cap. My family routinely goes over our monthly cap due to streaming, especially since the lockdowns. If you reach out to a Cox sales rep, they should be able to add 500 GB to your data cap for a fraction of the overage fees, which is what I do.
