2560x1440 blurry imac 5k

[PTechHD]

Alright, so I have a strange usecase here.

I have an Macbook pro M1 max 16"
I want to use my 2015 27" 5k iMac as a screen.

I installed OCLP to get the airplay receiver option to pop up on my iMac.
Now, I can easily stream to this iMac as a secondary extended monitor.

When no cable is plugged in, the max is 1080p.
When a cable is plugged in(its a USB-c to USB-a cable 10gbps data speed rated), it gives me the normal options for 1920x1080 (default) or 3840x2160. Now, this 3840 resolution is clear and great, but its too small. I need a 2560x1440 resoltion, but this is a low resolution for some reason. more blurry than the 1080 option. How do I get this as a clear image?

I would activate the "edit the system configuration of this display model" & "enable smooth scaling"
But this option is not available (its not even in the menu for this display) for this airplay monitor (even though its doing so over USB). Is there anyway to get a clean 2560x1440 image or enable this setting to get proper scaling