DLSS is what you’d need to upscale a 1080p image to a 4K screen. DSR goes the other way, so it downscales a 4K image to a 1080p screen - it’s basically like a super fancy anti-aliasing technique.
If you were then to send that 1080p image back to your 4K monitor, you’d still only have a (slightly sharper than normal) 1080p image. I guess the only way to know if it’s worth it is to test it out and see if the performance gains are worth the loss of resolution.
As AbAndyBar mentioned above, you would also need to reduce the resolution scaling in the game to actually get any improvement in framerate. Ideally Flight Sim would support DLSS and there would be no need for any of this fiddling around.
edit: I’ve been messing with this on another screen and I don’t see any obvious way to do it. The algorithm works to reduce your rendering resolution down to your screen res. But on a 4K screen a DSR factor of 2.25x boosts the rendering res to over 8K. You can’t render at a lower res and upscale it, you need DLSS for that.