Sounds like what the App "Home Connect Plus" (developed by Bosch) aimed to achieve, however they partnered with the device manufacturers.
As a business case however did not appear, the development and also the ability to use the App and the cross vendor integrations was shut down.
From a technical perspective, their approach however was interesting. They provided a separate adapter for each device they integrated and all of these adapters communicated with "Eclipse Ditto" via Amqp1.0 Protocol (other protocol like Mqtt or Apache Kafka would also be an option).
Eclipse Ditto, an open source digital twin Middleware, then provided an harmonized API for all integrated devices.
The mobile App would connect via WebSocket to Ditto and interact with devices of various vendors.
Technically, this worked well. Maybe something to learn from.
From a technical perspective, their approach however was interesting. They provided a separate adapter for each device they integrated and all of these adapters communicated with "Eclipse Ditto" via Amqp1.0 Protocol (other protocol like Mqtt or Apache Kafka would also be an option).
Eclipse Ditto, an open source digital twin Middleware, then provided an harmonized API for all integrated devices. The mobile App would connect via WebSocket to Ditto and interact with devices of various vendors.
Technically, this worked well. Maybe something to learn from.