Whoops, looks like I can already do this, but in the iOS app and the Mac app, pressing the large + button only goes up to 4 days. In the Mac app, if I use the stepper to the right of the number, I can make it higher. In the iOS app and Mac app, editing the number directly lets me set what I want.
So, there is a bug in the app that causes the delay to stop at 4 when you use the large + button.
(Not sure why I didn’t discover this earlier…I guess the large button looked like the only way to change the value.)