Hey Milk, I had a very similar issue years ago on rural property where the offending dogs were several hundred metres from me.
Unfortunately the owners are sometimes no further evolved than the dog, soooo.
Sounds like you've researched high frequency sound as a deterrent to the barking.
Based on nothing more than theory and trial/error I can advise the following;
The frequency that is effective on one dog may not effect another.
The effective frequency for said dog could be anywhere in the spectrum above 20 kHz.
This is why your off the shelf "birdhouse" sound repeller won't work, the frequency is fixed, don't waste your money, it is possible the dog may hear it but may cause nothing more than curiosity/more barking.
After wasting money, time and sleep on variants of 'ultrasonic sound repellents' I made my own and the results were impressive, after 2-3 days of continuous operation, silence.
Please see photo below.
I really didn't have high expectations for my "dog whisperer" project and to be honest felt like a mad scientist knocking up my contraption.
Please disregard the overall design of my unit as I had to project the sound through the bush to the neighbouring property.
All you need is the ability to change the frequency manually or automatically until you find the frequency most annoying to the dog in question, hundreds of options available as an app on your phone, easy.
Then a means of amplifying the sound to increase its effect on the dog, a very simple manually adjusted unit from Jaycar with a 3.5 mm jack for sound output and some cheap ass speakers, anything will do, you won't hear it.
The iPhones had the 3.5 mm jack then so you need a Bluetooth dongle or similar to connect the speakers.
In less than six pack you should find the sweet spot and neither the dog or the owner is any wiser!
Note, buy two six packs, you'll want to drink the other one if it works