Using WiFiManager for specifying config.h properties?

[bertrik]

Have you considered using WiFiManager for specifying the things that are currently configured in config.h?

WiFiManager creates a web server with a captive portal. You can connect to this using (for example) a mobile phone and enter the WiFi properties (AP name + password).
Along with the WiFi properties, you can also add a set of 'custom' fields, containing the MQTT properties for example. I can build an example for this, if you're interested.