Third party option are actually better than Sidecar, including for Windows (see my breakdown in recent thread about sidecar). To be honest I actually prefer Windows to MacOS, but I use both, and I use both my two 12.9 iPad pros (together with my 2 large Samsung tablets) as external monitors (with various apps, again most better than sidecar but not always free). Sadly I'm selling my old 2015 12.9, which I hoped to keep for life, even just for this feature, because it's developing a bright spot (the original 12.9 never had an issue but the one Apple swapped last year for the battery service is starting to show one, and I am afraid it will become like my 10.5). I am so mad at Apple for this, they oblige me to sell for cheap a device that is still very good, before it becomes impossible to sell / very annoying to use like the 10.5.