Another in place option is AES-encrypted ZIP. ZIP has the benefits that the Directory at the end of the file can also include piecewise metadata for full file validation.

A part of me wonders if the only real upgrade needed for the next "large file" KDBX relative format is from a GZIP stream of the XML plus attachments to a ZIP container of the XML with attachments in some folder structure combined with the choice of a good piecewise (modern) encryption for the ZIP container. (That is taking more cues from 7zip than from classic, now broken password-encrypted ZIP files.)

(Though as someone who tries to keep my KDBX files small, I think I'd still prefer the option of a whole-file encrypted format.)