I have thought about the project and came up with another approach which will certainly be feasible. Perhaps it is not as comfortable for a user as my first approach, but it should be perfectly fine for me.
The first part is the same - reading performance data from the Waterrower and receive HR via ANT+. But instead of sending the data to the wearable via ANT+, I could just save it on the controller (probably a Raspberry PI) and export activities in a suitable format for a) manual upload to Garmin connect or b) use a Garmin API if possible.
I would write a small android app (could be compiled to IOS as well when I use flutter) which acts as the user interface for the controller. I thought about using BLE for communication. The app could show if the controller receives HR information and of course display various sensor data. It would be used primarily for starting and stopping recording. In the future, it would be also interesting to create trainings which are then sent to the Waterrower computer.
I only need to receive ANT+ data which should nor be difficult to achieve. And it saves me the hassle of writing an Garmin IQ app, but instead I can use languages and frameworks I am familiar with.
Now only the hardware question is left. As mentioned, the target platform will most certainly be a Raspberry PI (v3 model B+). Could anyone recommend a compatible ANT+ hardware?