Oh! Good catch! I forgot about spent HD completely (we don't use them but I should add them for others...).I love this simplified sheet, nice work!
One comment: What about a place to track recovery Hit Dice? That changes from rest to rest, and from session to session.
Since dying typically resolves in a few rounds and doesn't really need to be tracked long term, perhaps replace those Death Save slots with number of Hit Dice available for rests?
Anyway, thanks for sharing!