Also interested in this. I invested heavily in the Fujifilm system a few years ago. Now have various lenses and an XT3 body. Frustrating that no Astrophotography software supports them but I could understand why if there was no public API.
However, I also started using Capture One Pro this year (Photoshop alternative) and it supports tethering. Really good app and I can control the XT3 from my laptop. Downsides are that the control is limited to manually initiated shots. I.e. you can change ISO, shutter speed and a few other settings, but you can’t set off a sequence of exposures from the laptop. You’d have to revert back to the camera controls for that, and you can’t do that whilst tethered. So either do all on camera, or sit at the laptop and press the virtual shutter release button every few minutes. I was hoping to leverage the excellent APSC sensor, but will probably have to invest in a dedicated astro CCD instead.