How can I delete ALL photos from my iPad and sync the photos from my iMac?

If all of the photos on your iPad were synced from your Mac, syncing again will overwrite the photo library on your iPad which would eliminate the photos on the device and replace them with the photos that you are selecting to sync.

If the photos are saved from websites, emails and/or were taken with the iPad, those photos must be deleted via the trash can. It may seem tedious but if your use the select and swipe up tip in the Apple support article, it will go much faster.

This explains how you can delete the photos from your iPad.

Delete photos on your iPhone, iPad, and iPod touch - Apple Support

If you are running Catalina or newer, you sync photos using the Finder.

Sync your photos manually using the Finder - Apple Support

If you are running older versions of MacOS, you sync photos using iTunes.

Use iTunes to sync your iPhone, iPad, or iPod with your computer - Apple Support