If you go back much further your client device might just have enough horsepower to run the game you're trying to play without streaming at all!
I Googled a little to see if the GTX 1660/Ti supported Vulkan Video and found this page from Nvidia[1] that says Pascal or later, so I wonder what the blocker is in this program.
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.