Some tips for improving WiFi performance;
There are other options. Google for WiFi repeaters / range extenders, WiFi mesh systems / pods.
Turning off 5 Ghz should not have any affect as they are different frequencies. You should be able to set up different SSIDs for 5 and 2.4 to connect as you need to.
It is also possible that you are actually connecting to 2.4 and getting the slower speeds even if it says 5 Ghz, I had that all the time with the same SSID.
The best is to have separate SSIDs to know exactly how you are connecting. Then you should try fixed, not Auto channels and channel width to see if you can find an optimal conditions.
A wifi analysis app might help identify good channels, but that information is very limited.
In the admin tool, Connection/Wifi/Edit no longer allows me to change channel selection from automatic to manual. Why is this tool being dumbed down? But it does allow me to turn 2.5 and 5 off and on. Meanwhile I'm using my iPhone to check for deadspots, and I may have found one where my Mac is located.
About the admin tool no longer providing the ability to make manual channel selections for 2.5GHz and 5Ghz bands: are the new gateways (XB6, XB7) able to sense traffic on channels and select the best ones?