Most wireless clients on airport express

Is there any way to "lock" certain devices to a certain access point on the network?

Unfortunately, no.

Or to set an order of preference?

Same answer.

This seems odd as most of those devices are physically closer to the superior Airport Extreme.

Most of these devices are likely iPhones, iPads, PCs or other small mobile devices, correct?

Mac computers will usually do a nice job of automatically switching access points to pick up the strongest signal, but iOS devices, most PCs and other mobile devices will not do this very well......if they do it at all.

You will need to get in the habit of temporarily turning off the WiFi on iPhone, iPads, etc when they move from one area to another, then turning the WiFi back on at the new location, and the device will usually......but not always.....then pick up the signal from the closest access point.

For what it's worth, my neighbor recently installed a newer "mesh" type of WiFi system at his home with three wireless access points, and he reports that all of his WiFi devices now do a much better job of automatically switching to pick up the strongest signal from the closest access point.

This seems odd as most of those devices are physically closer to the superior Airport Extreme. The Living Room 'Express' unit was only intended to cover a wifi dead spot on the far side of the house.

I would try to put actual numbers to your signal level from each router in each location.

You can download or use built in wifi analysers on most OS, but the most convenient to use is Mac Laptop as it has wireless diagnostics built in or android phone which has a well respected analyser.

It can be surprising but you might well find the signal level from the Express better than the Extreme in some locations even if the Extreme is physically closer. You can see which one is which via the BSSID.

My results show two routers and I am sitting equal distance between them.

On the 2.4ghz (SSID=ASUS)the signal levels are near identical. (It has also automatically chosen to use the same channel in both locations which is very odd).

But the 5ghz is very different. (SSID=ASUS_5G-1)

The unit on channel 48 has a received signal of -64db whereas the other router on 161 is received at -46db.

So sitting near enough halfway with one wall between each .. there is huge signal difference.

You cannot assume physical distance gives you signal level at all.

Although it might be less convenient to you.. one way to stop the clients joining the Express is to use a different wireless name on it. Since it is connected by ethernet you can use any wireless name. You will lose the roaming ability.. but you might prefer that other devices then connect to the right router.

If you happen to live in apartment building, that might also account for strange behaviour since you have much higher levels of wifi interference around you. The strongest signal is calculated at the antenna by subtracting the noise. If noise level is high it can give you effects which are unusual at first sight.

Perhaps the rest will settle down over time.

Not likely, unless you make some changes. Try powering off the Samsung TV, Google devices, Amazon, etc and then powering them back up to see if they will connect to the closest access point. If they won't, then you'll need to get with the support folks of these devices to ask them why the devices are not connecting to the stronger signal.

Newer iPhones and iPads seem to do a better job of switching access points, but If I move my older iPhone from my desk....where one AirPort Extreme is located....to the den, where another AirPort Extreme is located.....the iPhone will remain connected to the desk AirPort, even if it moves close to the den AirPort.