Some groundwork is needed before Quick Sync appears.
Firstly, the motherboard BIOS needs to be set to turn the integrated GPU (IGFX) on:
Then then your discrete GPU should be selected as the initial display:
Now, under recent versions of Clover this was enough on my Z170 machine but I have had to also do a bit more to get iMovie to launch without crashing immediately:
Invoke "Inject Intel" in config.plist while also injecting a headless ig-platform-id:
Be aware that my ID works for my i7-6700K but you'll need to find the correct ID for your i9-9700K.
I always try to use an SMBIOS that closely aligns with the actual CPU that is in use, for reasons of proper power management and, conceivably, other functions (like Quick Sync). Basically my approach is to show MacOS as much of the hardware as possible that is expected for a particular model. For a 9700K the SMBIOS should be iMac 19,1 but I'm sure you're not using that since the minimum OS for 19,1 is 10.14.4. You're in the same position I was in recently when faced with good reasons to move to Mojave but an inability to do so because of the GTX-1080ti. I ended up ditching the 1080ti and replacing it with a Vega 56 in order to make the move to Mojave. I suggest you do the same while switching to the correct SMBIOS. While it seems to be a severe downgrade the real picture is more complex, depending on the actual software that you're relying on. I documented it last November during the transition:
The Resolve tests were using the candle benchmark; the number refers to the number of nodes that could be active while achieving locked 24fps playback. You can see that there's an issue running the Luxmark Lobby scene on the nVidia card for some reason. If FCPX is your primary tool then I would consider my advice carefully because it's pretty clear that Apple is optimising its own software for AMD GPUs and Metal, which made a big jump in capability in Mojave.