Hi there.
Yes, unfortunately there’s no fully-baked Bluetooth library for Elixir yet. In regards to Blue Heron, the project is still very much a work in progress, and progress is slow, because no one is paid to work on it.
That said, Blue Heron can work on a Raspberry Pi 4, but it requires figuring out a few steps on your own, as not much attention has been paid to documentation. This is partly because it is simply not ready yet for widespread usage.
I am also not sure if Blue Heron can do what you want - as @lawik said, it is focused on BLE, which may not cover what you need. If you can link to an OBD Bluetooth adapter that you are considering, maybe I can give an informed answer of whether it can be possible with BLE.
There are tools in the Bluez family which can be used, depending on your use case. These are command line programs which can be invoked to interact with a Bluetooth chip. Probably easier than writing it in C, if it suits your needs.
I think USB might be easier. If the OBD USB adapter uses UART, you can use Circuits.UART, which is a mature project. I guess @ConnorRigby might now, I think he has done some stuff with OBD and Elixir.
There’s also a few other places to look at, like:
- Vehicular libraries/projects written using Elixir (e.g. OBD-II logger etc.) - #7 by pay64k
- Customize Your Car: An Adventure in Using Elixir and Nerves to Hack Y…
- GitHub - cboyer/serial-dispatch: Elixir/OTP application to read data from serial device and stream it to file and TCP clients.
I hope that gives you some inspiration for moving forward 
