Finally after the firmware fixes came about to improve cooling a few months ago and it was advised to just run the thing set up on its side, I decided to revisit running a 'desktop' on it.
Raspbian, as most people know, is 32-bit and the idea is you can plug the microsd card it into any model of pi ever made and it'll run. This sucks for performance and you can't run a version of firefox made the past few years. Chromium isn't an option on a 4GB ram system, it's just not.
I eventually settled on using sakaki's gentoo pi64 build and it was fairly painless to get running. Unfortunately, she hasn't kept the build up to date and there's a lot of weird customization she performed to get things running in the early days.
Anyways aside from having it run some compiles overnight and finding a new binhost to pull prebuilt stuff from, it runs pretty well. Actually better than my Intel Atom powered GPD Pocket (as it does not throttle ever). All the hardware works, and I've had no video issues since switching to the 5.4 kernel. It checks most of the boxes that OP asked for aside from the RAM being limited to 4GB. Unlike more powerful options on the market, it's got a wide release and a low price point so developers will be working on it.
So yeah, there's your AArch64 desktop you can buy today that everything works on for around $50. If you want to improve performance a bit more for IO, there's SSD/nvme adapters for it, and while I'm unsure about hooking into pci-e, the device has it...
Oh, and the audio thing is dumb as hell to complain about. Just output the audio through HDMI to your receiver and call it a day. What's the big friggin deal that the desktop lacks a 7.1 analog output. If you want high quality headphone audio, USB audio is practically plug and play on any OS these days. I don't get it.