Need help connecting 2013 imac as a 2nd monitor for my Win 11 PC...

I finally got the Wireless display feature working but 1) there's a noticeable but granted not a terrible lag and 2) the connection gets inconsistently disrupted...at times, it'll last a full day, other times, not even close.

So if you can think of a way to fix the above, I'd be curious. Otherwise, what option(s) do I have to connect them with cords/adapters?

As a second monitor to a PC my iMac with El Capitan [probably a bit older than yours] is a poor choice because of the computer inside it, has to process/reprocess the signal coming to it from the PC which can cause the slowness or lag.