Mobile developer for BLE sensor app (iOS + Android)

We make small BLE sensors that monitor temperature and humidity in warehouses and refrigerated trucks. Drivers and warehouse staff use our mobile app to connect to the sensors, collect readings and upload them to our cloud. We have a basic prototype, but the BLE part is not reliable enough for real use, and we want to rebuild the app properly for iOS and Android. What's going wrong with the prototype: - it connects fine to one or two sensors, but with 10+ sensors in range connections fail or time out - in the background (phone locked, app minimized) data collection stops, especially on iPhones and on Samsung and Xiaomi devices - downloading stored history from a sensor (up to 30 days) is slow and sometimes stops halfway - after a firmware update some sensors need to be paired again from scratch What we need: - an app for iOS and Android with a stable BLE layer that handles many sensors at once - reliable background scanning and data collection on both platforms - fast and resumable history download - firmware update over BLE (DFU) from inside the app - a short write-up of the architecture, so our team can maintain it We're open on the stack: native (Swift + Kotlin), React Native or Flutter. Tell us what you would choose for this project and why. You'll get several test sensors, firmware docs with the GATT profile and access to the prototype repo. Our firmware engineer is available for questions during the project. First milestone is stable connections and background collection on one platform of your choice. After that: the second platform, DFU, then offline mode for areas with no signal. Please start your proposal with the word "sensor" so we know you read this far.