That’s their huge drawback - expensive, usually clunky and suck at efficiency as a rule of thumb. Linear is not a necessary for our setups as many probably run with full Bluetooth/Wifi anyhow. USB has its own noise as does HDMI, so it’s sort of a losing battle however you add it up. I always shut the wifi/bt/hdmi off in my setups, but we live in loud environments and spending a ton of money to capture a couple more planes or messages a day is futility. That said some are perfectionists and it’s sometimes worth the conversation, especially since the OP is evidently experiencing such EMI issues.
Another way to quiet the power side of things down is to run from battery. Yet another expense and chain of supplies to get and add to the complexity, but it can be fun to experiment and tinker which is why many (most?) of us hang around here anyway I think.
ADD: Speaking of battery. I just ran some tests on one of my Pi4’s setup to boot directly from a USB stick, overclocked to 2Ghz, running an Argon Fan Hat with an Airspy Mini running 20MSPS, while running a sysbench test maxing out all 4 cores, the lowest voltage I seen from the below unit was 5.19v @ 1.58A:
The above is one of the more rare battery packs that you can buy that supports passthrough charging (IE plugged into the wall while being used). I have a few others, but all of them immediately dropped to about 4.8v or so under the same conditions. To make sure this wasn’t a fluke, I grabbed another identical battery pack from my other son and it tested the same. I also stuck my load tester on the 2nd port while running the sysbench and I was able to keep it above 5.17v @ 1.5a simultaneously.
This may be a good alternative for those looking to run clean power as it doesn’t get more linear than battery. Another advantage is that using something like this will provide you with around 12-14 hours of run-time should the power go out.