It's not that the Installer is bugged, it just wasn't designed for multiple different dGPUs since nvngx is Nvidia only. But you can try playing with the options below if they help, by setting to true.

; OptiScaler will try to force high performance GPU
; true or false - Default (auto) is false
PreferDedicatedGpu=auto

; OptiScaler will report only first high performance gpu
; true or false - Default (auto) is false
PreferFirstDedicatedGpu=auto

I tried those options, but it still shows the 6 GB of vram when I disable the original NVNGX

I also tried without success:

; Spoofed GPU name
; Default (auto) is NVIDIA GeForce RTX 4090
SpoofedGPUName = NVIDIA GeForce RTX 4090

; Enables Nvidia GPU spoofing for DXGI
; true or false - Default (auto) is true
Dxgi = true

; Skips DXGI GPU spoofing when caller method in the list (example: slInit|slGetPluginFunction|nvapi_QueryInterface)
; Be careful this will disable spoofing for rest of the calls also does not work on Linux
; method names seperated with pipe "|" - Default (auto) is disabled
DxgiBlacklist = auto

; Spoof amount of VRAM for DXGI
; Spoofed amount in GBs - Default (auto) is disabled
DxgiVRAM = 16

It is using the AMD card, not the Nvidia one, but the game seems to think that there is less vram than there actually is. That being said, the game does successfully use more than the amount of vram shown in settings, so it might be fine to just ignore the vram warning in the settings.

If it's directing to the proper GPU correctly and using VRAM properly, it's just a visual issue then at that point.

Could you send another log with the current options you're using?

I can think of a couple things that may help AFAIK.
1.
*I recall you can set a Nvidia GPU to be used primarily for PhysX.
2.
*In Win 11 (Win 10 menu may be different) Bring up Start Menu, Type 'Graphics settings', Select 'Graphics settings', Scroll down to the option to select the GPU AND/OR scroll to the game to and select the GPU you want the game to run. Multi GPU setups need to be configured to use the correct GPU sometimes.
*My laptop and IIRC desktop had a general preference on an older Windows Build around 22h2 where you could select the prefered GPU system default in 'Graphics settings' without having to set it for individual games. 24H2 on my desktop I no longer see the option without selecting a game.

P.S. To quickly get gaming without issue, I run DDU and swap my old to new graphics card, then install latest AMD/Nvidia Software/Drivers. PhysX games often run fine without Nvidia, though perhaps not all. I guess you need Cuda/physX for something.

It turns out, by adding fakenvapi and nukems (perhaps only the first is needed) on top of optiscaler, the VRAM is now properly detected as 16GB. So I guess optiscaler is best used together with fakenvapi then.