I did a mail too, that was it, no coupon and no banner. Had an AI chat write it for me, gave it last year's Koningsdag mail as the example and said do the same thing again. Opens were fine, nothing wrong with that part. But the sales didn't come on the day at all, they came in the week before, when the mail went out. Koningsdag itself was dead, same as yours.
So was it worth it. Basically, the mail yes, sort of, but not for the day itself; people buy when they read it, not when the party is.