I agree that the products ATT offer stink but 4 may indicate some other issue.
What the service tech would or should have done was to use their tablet computer to connect to the service point outside your home. They do simple speed tests to common web sites. The actual field techs don’t really have much training. They may be able to do a tdr on the line but if that is clean it would still be anything from the slice at the end of the block to any part of their network.

Unless you have some stable known source to provide a stream it might be hard to prove to them a problem.

I’d keep after them. Took me 2 years to get my line fixed and they had a hard time trying to fool me. They did try and I am sure they pull that stuff on customers all day long.

Yes, you can purchase your own device.

I think I’d be inclined to force them to fix it, Normal products tend to have a year warranty I’d think.

They don’t have to install it. Just have them send you one.

That kind of problem is usually caused by misbehaving drivers, but faulty hardware can also cause interrupts and deferred procedure calls to spike.
First thing I’d do is check to see if there’s a newer driver available for your ethernet and/or WiFi. Also make sure that no monitoring software (eg CPU frequency, temperature) is running on your system. On certain configurations, that kind of software can also increase DPC latency to the point that audio begins to crackle.