> activating the mic on bluetooth headsets [...] brings the audio bitrate down to a quality comparable to AM radio

This is actually an artifact of the BT profile used. It is the same for most bluetooth headsets (all I know), and the same for all operating systems I know. Usually most people don't realize because when they enable the mic they're listening to... well, other people (definitely not music) and human voice sounds quite good at these bitrates. Good operating systems will just pause the music when you switch to the headset profile so that you don't hear it heavily compressed (until you finish the call and disable the mic).

It won't matter which "BT version" your adapter is using. 2.0, 4.0, 4.2 or 5.0. The headset profiles have remained unchanged for decades.

There is something new in 5.x -- the "LE Audio" profiles -- but I know no headset using it yet. And then there's the myriad of propietary extensions. Most of them are snake oil; your mileage may vary. But then again when using proprietary extensions BT version does not really matter either.

I don't really care about the nitty gritty. The fact that it's even possible on something that's supposed to be high-end, is ridiculous.

I'm not so sure. I mainly used the airpods as a mic for Zoom calls so the content is voice either way. The quality degradation is drastic enough that I don't even try to use the Airpods on my computer for voice calls anymore. However, they're still my go-to for use with my phone and it doesn't seem nearly as bad there. Yes, they have different modes for mic on vs mic off but on my phone they are usable.

I'm curious as to why the same BT profile achieved 8kHz to my Big Sur Mac with a Bluetooth 4.0 chipset, 16 kHz to a Big Sur MacBook with a Bluetooth 4.2 chipset and 24 kHz to the iPhone 11 Pro with a 5.0 chipset (and the other custom Apple chips of course). Any ideas?

How do you measure that? Most likely this is some Apple propietary sheninangs so all bets are off. They are probably limiting things in software for reasons.

It is highly unlikely to be related to the BT version itself. There have been no bandwidth improvements to classic Bluetooth for decades. And Apple is unlikely to use LE Audio since BT 4.0 didn't had the required bandwidth for even full duplex 8kHz.

If you drag a Voice Memo to the finder you can find it under Get Info > More Info. This is handy because it syncs the memos from the iPhone back to the Mac for direct comparison. You can also see the it being negotiated in the debug console for both Mac and iPhone or in the Bluetooth Explorer audio graph for the Mac.

I've had a lot of problems with USB DACs as well. I frequently find that audio degrades into a kind of stuttering or static that doesn't go away until a reboot. I've experienced this with multiple DACs (Aune, Centrance, O2, and Presonus) on multiple machines (iMac 5K, 16" MacBook Pro, and hackintosh). I feel like it's worse in recent versions of macOS (say, High Sierra and later), but it's intermittent, so it's hard to say.

I always wondered whether Linux would become MacOS faster than MacOS could become Linux, but I honestly never expected it would be MacOS that would be the one with unrepairably buggy sound device quirks, no support for features of advertised display specs like MST, a condition that requires logic board replacemnt when you try to use the GPU after the battery fully discharges because of T2/BridgeOS, and power/USB problems that cause the computer to crash if you plug/unplug mouse and keyboard while docked over USB-C.

I fixed most of these things running Linux on my MacBook Pro, but if I didn't care whether or not my computer worked, I wouldn't have chosen Apple.

I had a long but with a FiiO USB Dac where it's drop the volume of one audio channel. I'd have to go into MIDI settings and even it back out.

Also experiencing similar headaches with MacBookPro and 3.5mm headphone jack, HDMI, and using Bluetooth midi devices - sounds crazy to try reading all these other problems with vanilla setups. It did use to “just work”.