Its been a while - but good things come to those who wait ;).
Trying to work out a new system you're unfamiliar with can be quite a challenge. In my case I got my first LoRaWAN concentrator along with some CubeCell HTCC-AB01 and tried to get them to work. It turned out - it was quite hit and miss. On the one hand, the firmware support for the RAK5146 with USB, GPS and LBT was not really ready yet - on the other hand, the CubeCell Arduino code has a fatal flaw with the preamble size so that those cannot join a LoRa network if used in EU868 MHz Band (the perfect fix by 1rabbit is linked as well!).
In the end, as I wanted to get this working as best as possible, I bought myself the RAK2287 Pi Hat and started modifying it. I was quite sure that the I2C signals should be available somewhere on that board - as well as 5V + 3v3 along with the raw PPS signal of the GPS module within the RAK5146. I was right and could bridge the PPS signal to an used RPi GPIO pin.
Using the I2C signal lines, Ground and 3v3 I added an I2C sensor interface (call it an ugly QWIIC connector ;)).
I installed the latest UDP Packet Forwarder package by Xose - and everything was working perfectly since then.
I even added brocaars Packet Multiplexer and started running a local ChirpStack instance on my home server. Now my sensors are feeding their data directly to my local InfluxDBv2 and Grafana - but at the same time my Gateway is still available for TTNv3 users to receive their data. Its awesome and with that I even receive my private data during WAN outages. Nice!
As added "bonus", my gpsTime project is running on the same RPi, using the GPS time of the RAK5146 and its PPS signal to be an extremely precise GPS timeserver in my network - and an additional BME280 is running as the "room sensor", because adding another battery operated device - if you are having more than enough CPU power (in form of the RPi ;)) is really not needed. The whole device fits behind the TV.
All in all, the project was very successful, I am working on some new ideas regarding the sensors, but this is pending on my KiCad 6 skills and deliveries of new RAK hardware currently on the way ;).
I still keep all infos in the balena Forums, so head over if you want more details.