On connecting to the tether over WiFi, the advantages over cellular data or Bluetooth is that a WiFi identity (MAC address, SSID) can be arbitrarily changed, and in fact are in consumer-grade hardware (yes, iOS uses a distinct MAC per connected network AFAIU, not positive of Android). This could be modified on every network connection, or even within a single session (requiring periodic reconnects). Other means of specific host identification via TCP/IP and 802.11 protocols are fairly limited.
On increasing workload, much surveillance is done via mass-produced hardware and software, and targets frequently-encountered devices (e.g., stock mobile phones, iOS, and Android systems). Adopting measures and methods other than these ... leaves a signature, but also means that specific new surveillance methods need to be devised for a specific target.
Also: in case anyone mistakes me for an expert on this area, I'm not. I've general familiarity with methods, techniques, protocols, devices, and operating systems.
If by "two problems" you mean that VOIP adds an additional problem, I don't quite grok it. It isn't a panacea, as you point out, but seems like a clear improvement.
Another advantage of VOIP is that you can easily obtain throwaway phone numbers.
> If you're making VOIP calls over a device that is itself connected to mobile networks ... you've still got the connectivity of the device itself to track. Presumably that's a long-lived relationship. At this point the information is limited to location data, but that, at the postal-code level is again sufficient to identify 90% of individuals within the US, based largely on residential and workplace locations.
Good point. They still don't know who I talk to and when, but they certainly can figure out who I am. I wonder how expensive the latter is, which I'd guess it depends on whether that analysis and the sharing of it is done automatically or takes a special request.
> The tether is connected over WiFi
I'm not sure that helps privacy: Wifi networks are likely shorter range than 5G cells, and the networks are well mapped. I suppose it does require involvement of someone with the map, but that might be easy to obtain.
> the MAC address space is already repetitive, and MAC addresses can be arbitrarily changed at the OS kernel level
I think iOS and Android randomize MAC addresses these days ?
> Burner phones on a one-use / short-use cycle would probably be preferable.
Yes, but a single burner phone, between the hardware and a one month plan, can cost $75-100. Using lots of them is out of reach for many people.