aruckle Adding the Mac to an iPad, No way.
Sidecar is expressedly intend for using the iPad as a second (or n-th) display for the Mac, not the other way around.
I use Sidecar for my mount simulator because the iPad Pro has a higher pixel per inch than even an LG 5K display that I have connected to my M1 Mini. This is to get the HFD of simulated stars to be as small as possible. The high end iPhones have even higher pixel densities, but the screens are abysmally small for use to display the simulator's output.
To add a second display to an iPad, you either need to (1) use AirPlay to a HDMI display/TV that is connected to an AppleTV, or (2) get a USB-C to HDMI adapter -- connect that to the USB-C connector of your iPad, then connect a HDMI display/TV to the adapter.
It does not work with older iPads that uses the Lightning connector, unless you have a Lightning-to-HDMI adapter, and those may be expensive and scarce (Apple used to sell one for about $50; I don't know if they still do).
USB-C-to-HDMI are a dime a dozen (well, not quite, perhaps $10 for a reasonably quality one, and you can find ones that have male connectors on both ends even).
That USB-C connector on the iPad is multi-purpose. It supports more than just charging. It is how I connect an iPad Pro directly to my home network. Gigabit ethernet adapters for USB-C are common.
There are Lightning-to-Ethernet adapters too for older iPads, but I have never been able to get much more than 100 Mbps through them even when they are advertised as Gigabit (and the Gigabit LED in the RJ-45 lights up).
Even a generation ago, an iPad pro is already close to a MacBook. With the M1 chip (ordered this morning? :-) the gap has shrunk even more.
If you want to control and view the Mac desktop from an iPad, just use VNC, as you do between any two computers in the past. VNC Viewer from RealVNC is a free app. You will need to run a VNC server on the Mac, and RealVNC has a VNC Server that runs on a Mac too.