So, the pseuso-SLC cache is still non-volatile. Even if you filled it up with sustained writes, and then immediately told the PC to shutdown, all that data is still there and "safe." The SSD would just work to flush out the pSLC cache to QLC storage once it's powered back up and basically idle. At least, that's my understanding of things, so the pSLC isn't a bad approach at all.
Note that with a 2TB SSD, the pSLC cache could be up to 500GB in size for a completely empty drive. So, if you could do sustained writes at max speed and fill that up, and then had to drain it at ~100 MB/s, it could take over 1.38 hours just to empty the pSLC to QLC. LOL. (Related: The drives take a while to recover in our Windows testing, unless you just wipe/format them.)
For the Steam Deck, the only way you'd really exceed even the reduced SSD write speed is if you had a wired connection. In my experience at least, you're otherwise limited by the Wi-Fi. It's a Wi-Fi 6 device (802.11ac 2x2 to be specific), which means best-case it has a theoretical 866 Mbps throughput. Except, you will NEVER see that in the real-world, even in ideal scenarios.
Basically, Wi-Fi 6 2x2 speeds will usually max out at maybe 550 Mbps, and if you have anything else happening on the network you'll probably get more like 350 Mbps. I have an 802.11ac 2x2 router for my house, and download speeds on the Steam Deck topped out at around 270–300 Mbps consistently. I get faster than that on some laptops with similar adapters, so it's likely the Steam Deck hardware that's slowing things down.
So if you have a hypothetical 200GB game download going, best-case it's writing to your SSD at maybe 60 MB/s, and very possibly closer to 35 MB/s. Even the worst QLC drives can basically do that all day without problems. Heck, even hard drives could do that, but HDDs are worse for power, size, and other performance aspects since they have that whole constantly spinning disk thing going on.
It would be interesting to try testing this. Like, a decent SSD and controller should write initially to the pSLC cache, but if it's only at ~40 MB/s, the cache can then be immediately flushed to QLC and would perhaps never fill up (until the SSD is completely full). The problem is that writing even 100GB of data at 40 MB/s takes a while, about 40 minutes. I guess that would be the question: if write speeds are slow, like sub-100 MB/s, do the SSDs even use their pSLC caches, or do they just write straight to TLC/QLC NAND?