The OEM horn is suboptimal, something more powerful is a justifiable, and a desired, safety upgrade.

I had some positive experience with installing horns on my BMW motorcycles hence a VW/Audi low frequency horn (VW part nr 18G951221) is a personal preference.

They pop up on ebay for ~$20, in addition you will need a proprietary OEM pigtail plug connector (VW part nr 4D097199), again ebay ~$7. One might get these at VW or Audi dealers, of course.

The beauty of this upgrade: no need for a relay, no errors in BMW on board computer.
Moreover, you will be able to scare truckers. Perhaps, I am slightly exacerbating, but the difference is quite amazing. In addition, the new horn fits into the same location as the original one.

Of note, the polarity of connections does not matter, which is also helpful. I cut the wiring about an inch away from the original horn, had to remove (blindly) a plastic zip fastener to extend the wires, soldered the new pigtail connector, protecting the soldered areas with a heat shrunk tubing. It is a bit tricky to mount the new horn in the original site but doable. Thin fingers are of help if you do not wish to remove the left side panel, etc.

It was fun project and the result is very satisfying indeed.