Answer for I feel I’m not getting enough speed/ Too much degradation

The AC53U seems like a decent router. However, if all the devices are operating on the 2.4 Ghz network, you lose a lot of the capability of the router since it is using the older protocol. The newer 5.4 Ghz devices can share bandwidth in a more efficient way so that one slow device on the frequency does not inhibit the ability of the other devices to run at maximum speed.
I would not be surprised to find out that the girlfriend’s device is an older device that is 802.11g or possibly even older. Older Wi-Fi devices can often wreak havoc on the throughput of all the other 2.4 Ghz devices on the network. I would suggest trying to use 5.4 Ghz on any devices that support that frequency and/or upgrade any Wi-Fi adapters if possible. You may also need to perform some speed testing on the devices when the other devices are out of the house or shut off. Then do speed testing on two devices at a time, then 3 together, and 4 together to try to isolate if one device is truly problematic. It may be that one device is malfunctioning and causing issues on the network and/or is an older Wi-Fi device that is bogging everyone else down.
Finally, it simply may be that if multiple users are pulling down Netflix feeds, then 30 Mbps may just not be enough bandwidth. Although 40 isn’t much of an upgrade. But if that is all you can get to try to relieve the bandwidth bottleneck, then it’s better than nothing.
Good luck!