This seems like a good job for Share Z-wave dongle over IP (USB over IP using ser2net / socat ) guide, though I think there has been some work to get OH to support this sort of thing natively but I’m not where where we are in the process.
Alternatively, you could set up MQTT 2.5 Event Bus and have an instance of OH running in the outbuilding that reports to your main OH over MQTT. It’s a long tutorial but it’s not all that difficult to set up for something like this if you are already familiar with MQTT.
In both cases you will need a separate Zwave controller in the outbuilding.
I don’t know of any way to boost the signal or extend an antenna to get through the walls and you’ve already tried everything else I would have tried.