Help to config dGPU on iGPU system

[Riquez]

Hi,
I've installed an rx6400 4GB card in my HP Prodesk 405 g6 (16GB, 4350ge)
My intention is to use the PC for Steam on livingroom TV exclusively.
It's working, sort of, but I have a number of issues to set it up correctly.

1) I cannot access BIOS when i connect HDMI to the dGPU - when booting there is no video output until the login screen of my OS (I have Dual-boot Win11 & Linux Manjaro - so whichever was used last time will boot direct to login)
If I connect HDMI to the iGPU, then I CAN see Grub/Bios screens.
If I connect HDMI to dGPU I CAN NOT see GRUB/BIOS - which is essential for dual boot.

2) my rx6400 is showing 512mb vram in Phoronix Test Suite

GRAPHICS:               Sapphire AMD Radeon RX 6400/6500 XT/6500M 512MB
    BAR1 / Visible vRAM:  512 MB                                            
    OpenGL:               4.6 Mesa 23.1.9-manjaro1.1 (LLVM 16.0.6 DRM 3.54) 
    Monitor:              SAMSUNG                                           
    Screen:               1920x1080

- it seems to me this is the vram set in BIOS. But since I can't view bios with the card, how can i change it?

Is there some way to set BIOS to use the dGPU only? I read it was "automatic", but it doesnt seem to be working well due to the issues above. My BIOS is S15 Ver. 02.14.00 & was updated to the latest a few days ago (before putting dGPU in)

My primary OS is Manjaro Linux. I do have Windows11 on dual-boot but I  don't know much about Windows & I don't intend to use it