I've been having the same situation since I got my new Docsis 3.1 modem. I have an Arris t25 and one that was sent by Comcast and they are all stuck in the ~300Mbps range.
Cable tech came by Monday and blessed my connection all the way to the exterior wall of my house, they will not enter the home to check the actual tap, because of covid19.
Spent HOURS on the phone, but no joy.
I tried your suggestion of reducing my service speed to 600Mbps. No joy. The speed is still 233.99 Mbps. They did say that the provisioning could take up to 24 hours to take, but I noticed that the modem was reset by Comcast shortly after the call and then again by me 10 minutes later. We cannot achieve 600Mbps