Yep I have noticed the same issue starting this weekend, at least that's when I first noticed it. I can run speed tests throughout the day and get anywhere from 10Mbps down to the advertised 150+, but mostly seems to be hanging around 20-30 and this is when nothing is going on, not streaming or downloading anything. Right now I just ran it 3 times and got 11, 23, and 30. I just went through a month and a half ordeal with Cox with constantly dropping connection, really don't feel like starting up another trouble ticket with them for this now ugh! I heard Optic Online is now taking orders for residential fiber here, looks like I may have to finally move on.
I checked your modem and all looks well but it appears most of your devices are on the 2.4Ghz wifi network which is not ideal for achieving your subscribed speeds. Can you try testing a wired connection and see what results you get that way?