Downgrading to version 15.5

So, I have accidentally upgraded my Imac 21.5 inch mid 2011 to 15.7 since it was the default and latest update without knowing it was a beta release without the needed KDK(and it's REALLY laggy.) And found out that the latest kdk would only be for 15.5 so far(from dortanias release page for kdks) and really wanna downgrade it to version 15.5 or Sonoma instead , but i don't know how to do it. I opened opencore, but when i click create os, it would just show some error from Apple and not show any versions. There isn't much on the computer, but i really need to complete my work soon without it continuously lagging, as it gets really annoying. Can someone please help guide me on how to downgrade it using OCLP? Ty!!