Building mobile app to control Smart appliances

Hi, I am looking to build an app to operate / execute smart home skills (Alexa, Google, Apple homekit) The app will control several smart home appliances, automatically. The app should be build in native code, preferably DART on Flutter. The triggers are sunset and sunrise times (I know how to produce/calculate these times, based on the geo coordinates of the phone) Since the skills should be executed not only when the phone is present, and it should cover all sorts of smart appliances brands, I thought about doing it with using Amazon Alexa, Google Home, and Apple HomeKit as the middleman or connecting element. So the app will "talk" to the Smart assistant and it will execute the skills (The user that I am building this app for has a range of devices, so it needs cover all types of smart lamps (tp link, philips, Lifx, Ikea) and also plugs/power-outlets (philips, tplink, Amazon basics) The skills are basically turning on and off several devices, selected by the user; for example: When sunset time is triggered the user can pre-select to switch-on several lights & appliances as well as switch-off other appliances or lock a door. Basically, the user should be able to select what will be switched on when sunset is triggered and what will be off. On the same way - the user can choose what will be switched off or on, when Sunrise is triggered. Ultimately, I would like the app to work on/with the popular 3 platform - Apple HomeKit, Amazon Alexa, Google Assistant Best, David