1. Your bird was probably preferable to my impulse as I read your story, which was to have said "Mind your own f&*%ing business!" in a no-holds-barred no-quarter-given tone of voice. So you're a better man than I am and can stop feeling so bad about it.
2. I used "On your left" for most of my cycling life, and it worked just fine. Calling it from far enough away and in a courteous tone of voice never seemed to raise any hackles. I bought a bell when I got the Crosscurrent S and have been using it instead. I prefer it, but would still use "on your left" without hesitation in many situations.
3. Now that you've had your moment of ire, gotten it out of your system, and discovered that you didn't like it, I bet you will suffer these fools patiently without any of us suggesting it.
4. As much as I hope all of us are good ambassadors for ebikes, there are those who just want to be jerks, and we will never convince them that ebikes are acceptable no matter how gentlemanly or ladylike we may approach them. For those, "Mind your own business" (perhaps without the aforementioned expletive) may be the best answer after all.