I've been streaming from an RX5700 XT which is something like 20x more powerful than my steam deck I'm trying to stream to.
20x is a huge exaggeration. It’s more like 5x in hypothetical flops.
For example, in Elden Ring an optimized 800p with mixed low to medium settings will hit 45 FPS.
On a 5700XT you’re looking at the high 90fps range for 1080p medium settings.
So it’s really in all practical purposes 2-3 times better and your steam deck has a maximum refresh rate of 90Hz anyway. On a screen that size you’ll barely see the difference between medium and ultra settings.
Something notable is that because of how math maths, 45FPS is a LOT better looking than 30FPS.
Finally, I’ll point out that anyone with hardware this old still has the option of using normal sunshine. Or you can just get a 6000 series card, the difference in cost between a 6600XT and a 5700XT is less than $50.
Using moonshine? As in what they list isn't a hard requirement just possibly the oldest they cared to test?