Wearable Accelerometer, SD Card, Bluetooth LE - Help Needed


I'm creating a sleep actigraphy system. I'm currently using existing smart watches and other wearables for data collection, but all of them have some limitations (some smart watches can't run apps in background, so can't track continuously; some don't have Bluetooth LE, and battery life is short; some manufacturers don't expose raw data, so have to use REST API for summary data instead of raw values). I'd like to create a simple device to log accelerometer data once per second. The data should be written to local storage, like an SD card, and later transferred to a connected Bluetooth LE device (e.g. iOS or Android device). I'd like to create something small enough to be worn on the wrist. I'm planning to house the assembled components in a 3D printed casing that can be attached to a watch band.

I've searched for projects where this has been done and have come up empty handed. Searching for components, it seems there are a number of small Arduino boards, accelerometers, bluetooth and SD card options, but I don't know what to choose or which are compatible. I've pre-ordered the Cortado board, and it seems like I just need to attach an SD card to it, but I'd like to build a prototype before the end of March. Can anyone help advise me in this project?

Thanks, James