So GUI and displaying the IP at the end is nice. But. I do wonder: If you already have to write a special image, why not just write the network config directly to wpa_supplicant.conf?
As evan__10 said, if you don't know the details ahead of time.
I own a company and we are currently developing a hardware tool for attendance tracking at child care centres. We use the Pi and our plans for shipping these things was to ask our customer what their SSID and password were ahead of time and hard code them in. This tool could solve our problem. Additionally, if the user changes their SSID or password at any time, the Pi won't be able to connect. We use a tool called Dataplicity to remote into the Pi and could change the password ahead of time, however if the user changes the password before we can do that, we lose all ability to connect to the unit.
Did you consider adding a password-changing webapp that connects via a smartphone's headphone jack? Or maybe even just with a _cheap_ piezo for bidirectional beeping to a smartphone? Consider the equalizer commonly used in the output path of a smartphone, but there should be existing work you might be able to build on.