I have a 27 iMac, a 14" MBP, an 11" iPad, and the iPad Mini 6 (just for context).
I think the Mini 6 is way too small to sidecar with the 27". I think it is too small to sidecar with the 14".
I think even the 11" is too small to sidecar with the 27", although I occasionally do so.
I think best would be a 12.9" iPad sidecar with a laptop, because the screens would be similar height.
I also find it very confusing having the Mac not be touchscreen, and the iPad being operated partly touchscreen (e.g., scrolling) and partly mouse/trackpad (e.g., selecting), especially when the iPad is normally 100% touchscreen. For me it takes a bit of mental gymnastics switching operating modes.
Sidecar is also sometimes glitchy and disconnects, unless hard wired.
In the end I find it is just more efficient to use desktop spaces gestures on the Macs. Sidecar, IMO, sounds better than it really is.
I am not sure I can recommend getting an iPad if the main purpose is to sidecar. A real second monitor would be better, IMO. Get the iPad to use as an iPad. Unless you really want the portability of the mini, the regular size iPad (10" to 11") is a far better size and value, IMO.
There are some applications like photoshop where you can use screen mirroring and then use the Apple Pencil on the iPad to use brushes etc., which kinda sorta works.
Sorry to sound less than enthusiastic about sidecar. I'm sure there are some specific use cases where it could work great, but I just haven't found them myself.
