Sequoia: 400 GB free storage but not really: How to force Photos to downsize using large encrypted disk images

I read Eclectic Light so I know about macOS APFS free space chaos . Recently macOS Photos quietly grew to consume most of my boot drive leaving only 30GB free. I switched to optimized storage while preserving a full local library on an external drive . So Finder showed 400GB free --- but it's "purgeable free". So not really. One of my sparse disk image ran out of space (causing cryptic error…