I would do it using ContentArea('ID') AMPScript function:
Build 7 separate content areas, containing HTML for the hero of the email, and select each of them, based on the weekday.
SET @today = Format(SystemDateToLocalDate(Now()), "dddd")
Then build IF / ELSE IF which switches between the different content areas, based on the value of @today
EDIT:
You might not even need to use a DE to map dates and content IDs. Imagine this scenario. You add a number field to your sendable DE, which you increment for each email sendout. This field is then read into the email, into the @JourneyDay - then you can use it to retrieve the content areas, named: Day1, Day2, Day3 and so on.
%%=ContentBlockByName(Concat("Content Builder\Day",@JourneyDay))=%%