191x7

Which motherboard? Which Bios version? MSConfig settings, did you perhaps use a dual-core before and use a setting to limit the system to two cores?

I would recommend the Sennheiser HD 560S. With the rest of the money you can get a different cable, a microphone, an AMP/DAC (like the Akliam PD4, Truthear Shio, Moodrop Dawn Pro, Fiio KA3, ...), a balanced cable, etc.

FurMark would be a stress test, and 3DMark Timespy would be a good indicator otherwise. Some people like Unigine heaven but I find it a bit outdated.

I have those Panasonic HJE 120s too, I had two and managed to lose one somehow.

That 3060 is what limits your performance on 1440p high refresh, the CPU just makes the frame variance higher than with a modern CPU.

Are the Simgot EW200 available? If not, you can go with the QKZ x HBB or the 7Hz Salnotes Zero 2.

Which use case? Music? Gaming? Phone? PC?

The CPU is your weakest point for MSFS. Get a Ryzen 5700X3D or 5800X3D. And a cooler for it if you're using the box AMD cooler.

Could it be coil whine?

Ryzen 9000 releases in July, and the 9000 X3D parts will probably come sometime after that, maybe early next year. I would recommend waiting till the 9000 series and deciding then. The 7800X3D will still probably be the best gaming option, but it might slightly drop in price.

Replace the GPU with a 7900GRE if you're playing on 1440p. Replace the mouse with something with fewer issues. Maybe the HyperX Pulsefire Haste Wireless if you like the shape.

Which cooler?

The 212Evo should do better than a stock AMD Wraith but it's also weak for a 5900X. The 5900X is the warmest, most power-drawing AMD AM4 CPU. My 360mm AIO was enough, but if I didn't have the AIO before purchasing the 5900X I would have gone with a 5800X or similar (I had a cooler in the range of the 212Evo).

Fix your cooling. And use a negative PBO offset of at least 15.

Are you on 1080p, 1440p or 4K? If you're on 1080p you need a Ryzen X3D CPU and an upgrade of the monitor to 1440p or 4K. Using a 4080 on anything lower than 1440p is a crime. If you're on 1440p or higher, you just need a Ryzen X3D like the 5700X3D or 5800X3D. If you're on 1080p and don't want to change, you need at least the 7800X3D (that would mean a new board and DDR5 RAM too).