Thank you for the link to this post.
It seems to describe a successful method to enable a splash screen from U-Boot on the iMX8 platform, although with LVDS rather than RGB.
I will spend some time understanding that method.
Inspecting Your Changes in U-Boot
I don’t think any of the U-Boot console commands allow inspecting the kernel config or device tree used to build U-Boot.
If I’m incorrect, could you provide a specific example?