I desperately need to get a good horn on my Rad City 4.
I added this one to my Mini ST2 a year or so back, in addition to the stock bell and a louder bell, so I have varying degrees of warning depending on circumstances:
https://amzn.to/3Rnd8iaMy little factory bell isn't very loud, but is perfect for quiet woods trails, the bigger dinger is for louder trails and bike lanes, but this will pierce a closed window on that car about to turn into your path.
It's rechargeable, but has a pretty large capacity and a charge lasts a LONG time (months) if you don't need the horn very often. It's extremely, annoyingly loud, which is sort of the point, and attaches easily. I have a USB charger on the end of the cord for the bike's main-battery charger, so I can charge this on the bike and never need to remove it. It could also be charged using the USB outlet on the bike's display.
I can recommend this and it's cheap enough to try without risking a lot of money.