All good suggestions - thanks guys.
Disabling it from the BIOS is a no-go on most laptops with this setup, as the 'onboard' graphics is actually onboard the CPU, so it can't really be disabled. Also the hybrid/switching setup is often done at a hardware level, primarily for power saving.

I did try disabling it from device manager, which while a bit rudimentary, is actually not a bad idea. FFXIV crashes on startup with onboard GPU disabled, which means it's still trying to utilise it.

It's frustrating because I'm certain the issue is the layer the launcher creates between launching FFXIV for real. I can set all the FFXIV executables to use the Nvidia GPU, but it doesn't matter - once I hit 'play' on the launcher, it does it's own thing. I can't launch FFXIV directly myself.