delete photos from iPad

You quite correct. The native Photos App does not provide a Select All option for mass-deletion of your photos.

However, there are third-party file manager Apps that do have the ability to mass-delete photos - with Select All capability.

Whilst there are several File Managers Apps in the Apple App Store from which to choose, my own “go-to” choice is File Browser - Document Manager by Stratospherix:

https://apps.apple.com/gb/app/filebrowser-document-manager/id364738545

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