With a 1 Gbps plan, the best speed you'll get is 900 Mbps. It's just the nature of networking with transmission control, error checking and retransmissions. This overhead will naturally consume about 10% of your throughput and it's engineered into your plan. Could your measurements be better? Yes, and hopefully the techs can tweak your neighborhood nodes.
Coaxial vs. fiber doesn't matter unless you really need the 1 Gbps upload speed of fiber. Cox can string my house with bailing wire as long as it's dependable, reliable and at the rate I expect.
I'm curious, however, if your speeds get better. Would you update?
So after the first tech guy, apparently another guy came to check on the box in front of my house. I wasn’t home the second time so I didn’t get to see what he did. I just did another speed test at 12 midnight, getting around 830~Mbps. It’s very likely to go down at peak hours but I’m hopeful haha.