Have you tried importing smaller "batches" of photos - instead of trying to import them all at the same time?
Importing/copy/moving more manageable batches may resolve the issue. Processing a particularly large numbers of files in a single operation can overwhelm the processing capacity of even the most powerful desktop computers, let alone a portable platform such as iPad.
When transferring bulk or large files, consider using a third-party file manager App - as these are often both more stable and capable than the native Files App.
Of those available, perhaps consider Stratopherix FileBrowser - Document Manager, or its “pro version” FileBrowser Professional:
https://apps.apple.com/gb/app/filebrowser-document-manager/id364738545
https://apps.apple.com/gb/app/filebrowser-professional/id854618029
You might review the extensive online Manual and help files to determine suitability for your needs.
https://www.stratospherix.com/products/filebrowserprofessional/filebrowser-professional-user-guide.php