Hi,
The first step is to determine what devices can be managed by the esp8266, I think you can manage most of them, just we need to identify and made the tests.
Next I need to get the components, that I don't have, I have several esp8266 modules (01,03,07,12E, dev board 0.9, dev board Lolin).
Also what we can think is to created a basic board with the esp8266 that manage the accelerometer, gps and the ws2812b and see if it can manage the sd card, and make and addon board that communicate with the esp to inform their capabilities (I/O, etc) and with that inform to central server.
The communication can go from a simple http api or using the MQTT.
For example,
1.- Share their total capabilities (the integrated+the addon) to the central server
2.- The server save/update the device in their data base
3.- The server will send alive commands to each device to see if they still alive
4.- When a device is going to shut down itself (in a normal way) it will send a disconnection message to the server.
3.- The client app constantly ask for a list devices and capabilities (if is a webpage the client will see appear and disappear devices dynamically)
The only think that I don't like is the requirement of the freelancer time tracking software, because you can only measure the time you stay in working in front of your PC, but to be clear it cannot capture the time you spend thinking on how to do the things or when you are making draws, etc.