Apple pen [Can iPad be used as a drawing tablet instead of Wacom?]

There are some subtle but important things about iPad features like Sidecar (and similar apps by Astropad, Duet, AirDisplay, etc.) to know whether they will work for you. Each option has trade-offs. You can use Sidecar to connect the iPad in two ways:

In Mirror mode, you can use the Apple Pencil on the iPad to work in Mac Photoshop, and you will see what you’re doing on both the Mac and iPad displays. Because the Mac and iPad must show the same thing in Mirror mode, the resolution of the desktop on your Mac display might drop, and the image might change (black bars on the sides), to fit what the iPad is capable of.

In Extended Desktop mode, the iPad will act like a second display, which can show different windows than the main Mac display, with both displays operating at full resolution. In Extended Desktop mode, if you want to use the Apple Pencil to control Mac Photoshop, you must move the Photoshop application frame to the iPad and edit on that (the Mac display would not be involved), because in Extended Desktop mode, the Apple Pencil can’t cross over from the iPad to the Mac screen.

In those two ways, Sidecar works like a Wacom tablet that has a built-in display. There are also less expensive Wacom tablets that don’t have a built-in display. Those work more like a mouse or trackpad, so you would buy one of those if you wanted to use a pen across one or more Mac displays connected using Extended Desktop mode.

The other option that was suggested is, instead of using the iPad as a Mac display, use the iPad version of Photoshop and edit using the Apple Pencil in that. This can work great, because Photoshop on both Mac and iPad sync documents in the cloud seamlessly and with no manual uploading/downloading. For example, you can start a document on the iPad, and then in one click in the Home screen in Mac Photoshop, open the same document to finish it. Another reason this is great is that if you are working in iPad Photoshop, you don’t need to have the Mac around. So what’s the trade-off with this option? Many features of Mac/Windows Photoshop are still missing in the iPad version of Photoshop. So if you want to use Apple Pencil on your iPad to edit on your Mac, and you often need features that are not in Photoshop for iPad, use Mac Photoshop through Sidecar (or similar apps).

There's also the Astropad Studio app. $100/year.

https://astropad.com/product/studio/

I would explore Photoshop on the iPad first though

https://helpx.adobe.com/au/photoshop/using/photoshop-ipad-faq.html

You might be able to use the sidecar function if your computer and iPad meet the requirements. Here's more info. https://support.apple.com/en-us/HT210380

You can also use Photoshop on the iPad for many editing functions. Save your files to cloud docs so they can be shared between the iPad and desktop.