Transfer from my iPad Pro to our new iMac?

Transfer what exactly?

You cannot use migration assistant between an iPad and a Mac.

Your contacts, notes, and calendar if stored in iCloud or a third party email account can be accessed by adding the relevant iCloud or Email account on the iMac, in System Settings ➜ Internet Accounts.

For things like Photos, if you are not using iCloud Photos, you can manually transfer them by following the steps below.

click here ➜ Transfer photos and videos from your iPhone or iPad to your Mac or PC - Apple Support

When using Set Up Assistant on the Mac you can only transfer from another Mac or Windows PC. The information from your iPad will be transferred on the next step of Set Up when you sign in to your Apple ID that you used on your iPad. This will then transfer any information that you have saved to iCloud. To make sure that information gets transferred, you will want to be using iCloud on your iPad for storing Contacts or Messages if you want those transferred. If you used iCloud Photos on your iPad, then those pictures will be transferred to your Mac as well.

Other information that is saved in iCloud is:

• iCloud Mail
• iCloud Drive for files and folders
• Calendars, Notes, and Reminders
• Safari Bookmarks
• Stocks
• Weather Settings

All this information will be used when signing into your Apple ID on your Mac.

When you use your Apple ID on your Mac that you used on your iPad, the information that was stored there will automatically be available. To verify what is stored in iCloud on your iPad, go to Settings > [name on top] > iCloud. Any info that is turned on there will be available on your Mac.

When setting up your Mac, skip the step to transfer data and proceed by signing in with your Apple ID.

Just log in to your iCloud account on the Mac, in System Settings ➜ Apple ID and turn on the options for iCloud data in the iCloud section there for things ike contacts, email, calendar, notes etc...

If you are using iCloud Photos on the iPad, you can turn on the iCloud Photos option there too, and the photos will appear in the Photos app on the Mac.