I'm attempting to pilot a research experiment whereby 30 "recruits" will wear a fitness tracker 24/7 for a month, and at the end I will analyze their granular (minute-by-minute, ideally) step/distance/heart rate/sleep data to arrive at a conclusion about their activity. The issue is that these "recruits" lack access to consistent cell service, and will only be able to sync with a smartphone once a week; and, when they do, all "recruits" will have to sync to the same smartphone.
My question is, what's the best fitness tracker/software option for this project? In short, I require a Bluetooth fitness tracker (with steps, heart rate, and sleep features) that can locally store at least a week of granular fitness data, with accompanying Android software that will allow multiple devices to automatically upload data to the Cloud via the same phone. I don't require access to any GUI on the phone or a screen on the fitness tracker; those being tracked do not desire on-demand access to their own data.
A secondary concern is device/software robustness, as my capacity to fix equipment/software problems in the "field" will be limited.
Fitbit, for example, is a decent candidate, as their Partner API allows access to intraday data and all data can be accessed via their Web API. However, only one person's fitness data can be synced on a given account, so this isn't a possibility, as uploading data would require the cumbersome manual effort of signing into and out of thirty accounts in a row.
I've looked into Google Fit as an option, but every fitness tracker I've found (Fitbit, Garmin, Huawei, Xiaomi) uses a proprietary app that you're forced to interface with via Google Fit, removing the possibility of tracking multiple people with the same device.
Thanks in advance for your input; I've spent significant time researching the issue, so I hope my question is suitable.