Didn't read the entire thread but here's my quick list of how to get good internet speeds over the entire house:
1) Have one of your router or access points as high as possible.
2) Check the 'sync' speed on your router settings; if they're NOT near what line speed you're paying for: disconnect everything else from your phone sockets if possible, in fact rip them all out for best results and just connect your router to the master socket if you really want best results.
3) Split 2.54GHz and 5GHz SSID's.
4) Download WiFi analyzer app and see what channels are free based on how congested your neighbourhood is. Often the 'auto' setting will get this right but you should validate. Select a channel with fewer other access points if possible. (You will see channels 1, 6, 11 populated mostly) This only really applies to 2.4GHz band as 5GHz band using 80MHz channel bandwidth occupies all UNII1 frequencies anyway. (ignore this if you're not sure what I just said)
5) Run speedtest either on a LAN connection or a 5GHz WLAN device located near the router to check for good speeds.
6) Convince anyone possible to connect to the 5GHz network, not the 2.4GHz network as it has higher bandwidth. Leave the 2.4GHz network for devices that struggle with connectivity in dark areas.
The above is good for anyone. The below is for those that want to extend their WiFi.
7) Buy a pair of powerline adapters. Don't fall for big numbers! If your broadband is 100Mbps, there's zero point buying a 1000Mbps adapter. Mine are all TP-LInk 600Mbps. Pair them, plug one in near to router, connect ethernet cable.
8.) Pick up an old router from facebook marketplace; most people have an old one sat around anyway. Situate this and the other powerline adapter as far from the router as possible but in a useful area to make best use of the additional coverage. Look online for how to configure your spare router as a non-DHCP access point. The Sky Q Hub or the BT Hub One are both fine. It may need trial and error in what order you set the wireless name, remove IP setting, and turn DHCP off. You do want the wireless SSID names to be the same as your router to ensure seamless movement around the house! (note: a small switchover period is normal)
That's it. Sounds complicated but really isn't and is in my opinion by far the best way to 'extend' WiFi signal - way better than boosters and the like and costs less too - plus you can simply move it any point later. I have my extended WiFi in a log cabin at the end of my garden with 30Mbps download speeds - that's due to the AC mains cable distance. But really that's good for most things people need. The Sky Q hub has been set up as an access point for about 16 months now without skipping a beat. The powerline adapters should be problem free but can reset on power cuts. Just something to be aware of.