I've been promoted several times for adding business value, complex or simple in implementation.

Saving every team from doing X manually, saving so and so many engineer hours per year. At cost Y per year instead in one team. The higher X and lower Y the better.

Backups did not exist. Now they do and work every time.

I wrote the top customer appreciated feature, associated with an increase with X increase in whatever metric.

"It was really hard" shouldn't come into it, or not as much by far.