The app already had the code for "isolate the puzzle and do perspective correction" so the uploaded images all looked something like this: https://magicsudoku.com/example-uploaded-image.png

By "slicing in bulk" I mean the server was the one that split that out into 81 smaller images rather than the app doing the slicing and uploading 81 small images.

Taking them from different angles was done because the perspective correction adds distortions that I didn't want my model to be sensitive to.