So... I think I've managed to come up with a workaround, an hour after posting this and 6 months after first trying to fix it.
I'll write it down here, because it's been insanely frustrating to not find any info anywhere on this, so someone else might come across this.
Basically, it looks like the sound bar (probably all sound bars) are incapable of feeding back the unique properties of the TV panel. For example the new firmware on this TV enabled G-Sync support, but that was completely unavailable through the sound bar... probably for other reasons but it just wasn't detected. But it isn't capable of being told by the TV that it supports 120Hz 1440p, so it's just impossible to run that format through the sound bar.
Anyway, this is the workaround. It'll differ based on which video card your PC has.
Run a HDMI cable from the GPU (HDMI port) direct to the TV, that'll unlock the full capability of the TV. G-Sync still doesn't work properly but I'm going to give up on that.
Then, buy a Display Port to HDMI adaptor. Plug that into a display port on your video card. Then run HDMI cable from that direct to the sound bar.
So you'll have two cables going out from the GPU, one to the TV and one to the sound bar. The PC will pick up the sound bar as a third monitor/display, but also a digital audio device. You can then change the default audio device to be this third monitor which will have full Dolby Atmos support into the sound bar!
So you can have 120Hz 1440p playing out from the native HDMI port on the video card, and dolby atmos playing out to the sound bar through the display port to HDMI adaptor.
Bit of a messy solution which results in more cables, you lose the ability to control the sound bar with the TV remote, you lose sound settings from the TV, you gain this extra third monitor which isn't really a monitor, but I can accept all that as now I've got Dolby Atmos and the full potential of the TV now.
Really quite frustrated that this isn't documented anywhere like I'm the only one who's ever tried to do this, but whatever. Hope this helps someone in the future.