Downgrading iMac?

Having an issue here. I have a 2015 5k retina iMac. I also have a M1 Macbook pro 16"

I was looking for ways to use the iMac (and my 21" 2010 iMac) as screens for my macbook. tried everything and eventually installed OCLP trying to use a feature that I can't actually use. Turns out, the 27" 5k iMac was capable of just using airplay as a receiver with a higher resolution than 1080p if I just ran a USB-c cable to my Macbook from the iMac (fast data cable).

I've been using this and also an HDMI capture card (to the iMac and using obs studio). Been going back and forth between the two.

Issue is, I updated my Macbook to Sequoia last week. I have not been able to use the iMac as an airplay receiver since then. It simply won't extend or miror. it just says its cant connect after a minute or so. Thinking foolishly, I updated the iMac thinking maybe it just needed to be on a newer OS (it was on a much earlier OS as I never used OCLP to update the OS).

But now, OBS will not show any resolutions higher than 1080p even though I used to be able to select a 4k resolution. and the airplay still wont work smh.

Any ideas?

when I first updated, I had very memory intensive processes hogging up my imac as soon as OBS started. and the background was solid white on the desktop. Almost unsuable iMac. Realized i needed to update OCLP and install the root again and did that.

But for some reason, OBS is only detecting a max output of 1080p on this computer for some reason. Does the same on my 21" iMac, but this is a 27" 5k iMac that used to allow me to select a 4k resolution in OBS as long as the capture card was a 4k capture card.

Sorry for rambling, im just trying to find any solution. to either get back my OBS higher resolution or make airplay work again