I did a 15 yard zero. Ballistically, for 9mm, it is a great zero. Not off that much even at 100 yards... See:
I used to zero my rifles for 50 yards. And, that is the distance I prefer to shoot at. But, I have to go to an out of town range, 1 hour away, to do that. And, one of the reasons I got the 9mm carbines was because I could shoot them at my local indoor range.
And, a 50 yard zero for 9mm is MUCH further off at 100 yards. 15 yard zero - you don't really start getting more than 1 inch off until you get past 80 yards. When I got my black CX4 in the summer of 2020, I started doing a lot of reading on what was the best distance to zero. If I remember right, I think the only better one than 15 yards was a 12 yard zero. Someone posted the numbers on a forum that I found when I was doing google searches. But, I didn't want to shoot that close all the time once I zeroed the gun.
Yes, shooting at 15 yards is very easy. It is more challenging at 50 yards, and that would be my preference to shoot. If the outdoor range wasn't so far away, I'd probably go with the 50 yard zero, even if it does drop like 8 inches at 100 yards.