Old AirPort base stations to extend 3rd party Wi-Fi via Roaming Network Ethernet

First I dug out the 2011 AirPort Extreme 802.11n 5th Generation and updated its firmware ... only to then google that new FW versions have disabled the "5 GHz -only" feature (maybe because too many people then complained that old 2.4 GHz-only gear could not connect anymore?).

This information is not accurate. The 5th Gen AirPort Extreme produces dual 2.4 GHz and 5 GHz bands. It is not possible to disable the 2.4 GHz band (or the 5 GHz band for that matter.

If you had the 6th Gen AirPort Extreme......also known as the 802.11ac AirPort Extreme......it is possible to disable either the 2.4 GHz or the 5 GHz band on that model.

It is possible to assign a different wireless network name for the 2.4 GHz and 5 GHz bands on the 5th Gen AirPort that you have, so if you wanted to only use the AirPort Extreme with 5 GHz capable devices, you could "tell" devices to only connect to the 5 GHz network. In this case, devices would "ignore" the 2.4 GHz network since they are connected to the 5 GHz network.

Let me start with your second question first:

Matti Haveri wrote:

-> Question: 2004 AirPort Extreme 802.11g Snow Dual Ethernet Base Station does still seem to work but would it produce unexpected glitches with more current gear?!

Definitely not worth it. First 802.11g only runs on the 2.4 GHz band. If you are planning on setting up a 5 GHz-only network, you will need to run to separate networks for each band. Sort of defeats the purpose of disabling the 2.4 band on the AirPort Extreme. You also don't want to run the slower 802.11b/g clients on your wireless network as this will affect it overall performance. Finally, I believe, that this old base station does not support WPA2 so wireless security will potentially be compromised. Pretty much a "museum piece" now.

-> ...but is it counter-productive and asking for trouble to use many Ethernet-connected (and old!) base stations to extend Wi-Fi also to the backyard? Is the 2004 AirPort Extreme 802.11g Snow Dual Ethernet Base Station too old for that?

Multiple base station connected by Ethernet is actually more efficient performance-wise, but again, not with the 802.11g model. Stick with all "n" (better still "ac") models for a roaming network.

-> Question: I noticed that AirPort Utility iOS 15.4.1 lists FW versions 7.5.2-7.8.1. Is it really possible to downgrade the AirPort base station and do those versions have 5 GHz-only option?? (It seems my other gear does not support 5 GHz so this might be a futile exercise anyway...).

Unfortunately, I don't have a 5th gen AirPort Extreme to confirm, but my (last remaining) 802.11ac Time Capsule, running 7.9.1 can be configured for a 5 GHz-only wireless network. Bob Timmons is the resident AirPort expert here. I'm sure if he chimes in, he can confirm that with you for your model.

-> Lately my old 2010 Apple Magic Mouse bluetooth quite often disconnects from Mac mini 2018 and it was recommended to switch Wi-Fi to 5 GHz -only to avoid interference. But the old Huawei supports only 2.4 GHz.

It has been my experience that this issue only occurs if the mouse & wireless access point are near each other. Think both on the same desk. Otherwise, this really should happen.

FWIW, I currently use a Logitech wireless mouse/keyboard combo that either uses a proprietary "Unifying" or a Bluetooth network for connectivity. I opted for the former, which requires a single small USB dongle (one comes with each device) to be installed on my Mac for the connection between devices. As such, Wi-Fi interference is not an issue for me.

Matti Haveri wrote:

Reportedly the Mac mini 2018 bluetooth is also quite sensitive to poorly shielded 3rd party USB devices.

Interesting, as I currently have a 2018 Mac mini as well, and don't seem to have any issues in this regard. Then again, I don't use a Magic Mouse, and use a Logitech one instead, as I mentioned earlier.

BTW the Huawei is somewhat further away behind a few wooden walls upstairs but NetSpot.app still reports slightly better 2.4 GHz SNR compared to the AirPort (but the devices connect OK to the AirPort 5 GHz which is the only 5 GHz in the neighbourhood).

As I'm sure you are already aware, due to physics, the 2.4 GHz band can traverse object better than the 5 GHz band ... and why, the higher band is basically only useable within the same room as it is being broadcasted in. Remember, Wi-Fi is a two-way communication. Even if you have a very strong signal coming from the wireless access point (WAP,) your wireless client must be able to "talk back" with sufficient strength to be "heard" by that WAP.

This, of course, shouldn't be an issue with a properly configured roaming network where you have strategically placed your WAPs for the coverage area you require.

Matti Haveri wrote:

... while there seems to be no such options for the AirPort. The 2.4 GHz channels are are automatically set, currently to 1 and 11.

Yep, one of the reasons I switched to other networking hardware solutions ... more options/control. Especially since Apple got out of the networking hardware business in early 2018.

Anyway, the current setup seems to work pretty well with the exact same SSID "Network Name" and password and "WPA2 Personal" security settings for both base stations.

This is the setting that most users should use, and coincidentally it is also the default setting for the AirPort routers.

I would only use the option to assign a separate name to the 5 GHz network if all of the WiFi devices will always be located in close proximity to the AirPort Extreme. The reason.......if you have separate names assigned to the 2.4 GHz and 5 GHz networks, then devices that are currently connected to the 5 GHz band will not automatically switch over to the 2.4 GHz band when they move further away from the AirPort Extreme. And.....devices that are assigned to the 2.4 GHz network will not automatically switch over to the 5 GHz band if they move closer to the AirPort Extreme.

Nothing is ever perfect on a Wi-Fi network......the best that you can do is hope for a best compromise situation.