I see that you've decided to use POST for entry-editing (presumably by way of wholesale replacement). Have you considered allowing PATCH, which is intended for the purpose? I realise you'd have to either figure out how to apply application/json-patch+json to some kind of JSON representation of an entry, or break consistency and use text/x-patch as input, but it might be worth it for small edits on large entries.
/api/v1/journals/{username}/entries/{id}