There is one other USB DAC for the Xbox Series X (besides the Astro Mixamp) that I only just became aware of. It's a relatively recent release called the SteelSeries GameDAC Gen 2. It comes as a base station for Arctis Nova Pro headsets, with wired and wireless variations. Unfortunately the GameDAC Gen 2 cannot be purchased separately. You also need to make sure you're buying the Xbox version, as there are Xbox and non-Xbox versions of both.
The interesting thing about this product to me is that it has a Line-Out jack. Both the wired and wireless versions have this line level jack. The wired version is cheaper and seems to have a superior DAC, so I got that one. As mentioned above, it comes with a pair of headphones, but they're not very good. If I was going to use the headphone amp in the GameDAC I would use different headphones.
Anyway, back to the Line-Out: running the Line-Out into my audio interface, it proved to be pretty decent. There was a very slight channel mismatch on the level meters, but nothing audible. At first I was dismayed by a terrible noise floor, but it turned out to be some kind of earth loop that was addressed by disconnecting my audio interface from my PC. With that out of the way, the noise floor was acceptably low.
I obviously haven't had the unit for long enough to see whether it has the same kind of deal-breaking USB bugs as the Mixamp, but so far it seems pretty reliable.
But here's the (possible) kicker: Somehow, SteelSeries have managed to generate ~50ms latency in this device.
The 50ms might not be a deal breaker for everyone, but for me it is. I use both wired and wireless headphones and for the latter I use an AptX LL bluetooth transmitter. The combined latency of that + the gamedac becomes unacceptable.
I think it's absurd that a wired device targeted squarely at gaming can introduce so much latency.
So yes, the Steelseries GameDAC Gen 2 is a potential option for USB-to-analogue DAC for the Xbox Series X/S, but to get it you have to buy the whole Arctis Nova Pro package, and be aware of the abnormally high latency for a wired DAC.