Importing photos to your iPad from SD (or other media) is certainly supported. While larger photo collections may take considerable time to transfer, the process should import compatible photo formats without issue. The native import process copies the files on the card; they are not moved or deleted.
Start here:
Connect external storage devices to iPad - Apple Support
Automatic import of photos from your camera SD Card to your Photo library has a couple of technical caveats:
- the SD Card must be correctly formatted as FAT, FAT32 or exFAT (FAT64)
- stored photos are stored within the expected DCIM folder
Import photos using Apple camera adapters - Apple Support
If you are unable to use the direct photo import feature, you can still manually access and copy photos from the SD Card using the iPad’s built-in Files App. To do so, just open the Files App - and in the Sidebar you will see your SD Card in the list of Locations. If necessary, open the sidebar by tapping the sidebar icon at top-left of the Files App Window. Photos can be copied to the iPad Photo Library, or copied to a folder of your choice either on your iPad or elsewhere.
Using a third-party file manager App may be more flexible and might also ease the process. Of those available, perhaps consider Stratopherix FileBrowser - Document Manager, or its “pro version” FileBrowser Professional:
https://apps.apple.com/app/filebrowser-professional/id854618029
https://apps.apple.com/app/filebrowser-document-manager/id364738545
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
Speaking personally, I have found both to be very reliable - and significantly more flexible than the native Files App.