So I've retested SN570 in the Steam test and it really is SUPER slow.
It starts out fine, and then speeds fall of a cliff, while the drive is still 100% busy. This confirms that the issue is not with the host PC, but rather with how the drive reacts to Steam's weird pattern of read-write activity (which is EXTREMELY unoptimized for the task). Still, it is what it is, and other drives do much better here. Remember, the SLC cache is tiny at 16 GB, that probably plays a role here.
I do remember updating the firmware in Jan 2023 before retesting all drives, with new tests and new platform, so maybe that's the reason the numbers were better in the past.
Here's a bit further into the test. The drive is still sitting at 100% busy, and write speeds are extremely low, even though they fluctuate up a bit, from time to time.
I tried
@Sunlight91's tip with the write cache, and indeed it helps a lot to improve things.
Uncheck: "Enable write caching on the device". While this certainly hurts performance in other scenarios, it greatly helps for Steam. Now the unpacking completes in 92 seconds. Comparable to other NVMe drives in the test group.