Phantom Gaming B850I Lightning Dual GPU

Hello, I'm Yuns92 and new to this forum. Unfortunately, my English isn't perfect, but I hope everyone can understand me.

I'm having two strange issues with my new HTPC build:
Components:
- Case: Fractal Design Ridge (White, PCIe 4.0, Mini-ITX)
- PSU: Seasonic FOCUS SGX-750 (750W SFX)
- Motherboard: ASRock Phantom Gaming B850I Lightning WiFi
- CPU: AMD Ryzen 7 8700G (boxed)
- RAM: Klevv Fit V 32GB Kit DDR5-6000 (CL28-36-36-76)
- dGPU: Sapphire Pulse Radeon RX 6800 (16GB GDDR6)
- CPU Cooler: Thermalright AXP120-X67 White ARGB

Note: I delidded the CPU, applied liquid metal for better thermals, and insulated the exposed SMD components on the CPU with nail polish to prevent short circuits.

After a fresh Windows 11 install (fully updated), I enabled the EXPO profile in BIOS for RAM overclocking. I have also updated to the latest BIOS version available.

My goal: A dual-GPU setup using "lossless scaling" (dGPU renders games, iGPU handles frame generation).

Problem 1:
When using the motherboard's HDMI output (iGPU), I cannot select 4K@120Hz on my TV. This is unexpected since both the motherboard and 8700G's iGPU support HDMI 2.1. Works flawlessly via the dGPU's HDMI port.

Problem 2 (more critical):
If my TV is connected to the motherboard (iGPU) and I set the dGPU as the default "High-Performance GPU" in Windows, any game launch causes a full system freeze (games, Task Manager, all apps). A hard reset is required.

However:
-> Using the iGPU as default (display connected to motherboard) -> games run fine.
-> Using the dGPU as default (display connected to dGPU) -> games run fine.

Troubleshooting done (no success):
- Updated BIOS to the latest version.
- BIOS reset to defaults.
- Use default speed for RAM.
- Set PCIe Gen to 3.0 in BIOS.
- Increased iGPU VRAM allocation.
- Reinstalled AMD chipset + GPU drivers.

I'm out of ideas. Has anyone encountered this or know a solution?