regardless of which developer you engage, you should be aware of possible issues and restrictions from browsers and operating systems against allowing tracking in the background, which it seems that you will require for this spec. If you are creating native mobile apps, you will need customisation to make the apps perform this function, so your developer should have skills in this also.
Please update once your get time. I chat with multiple developers but all are occupied with other projects.
Regarding gps location, if current una app can take gps location while check in and check out of attandance and while taking pic to submit the task. This much specification will be enough for field services.
We will hire app developer to create app in react native later on. But we need the module first for attendence and project module for task submission.