LTDC data starvation during simultaneous SD-RAM and NOR Flash access with STM32F429 rev 3
I also had this issue using an STM32F767 driving a 800x480 color LCD, also using concurrent DMA2D transfers into the framebuffer with only one bank of SDRAM available.  I solved it by running the SDRAM as fast as the data sheet would allow (HCLK at 168MHz) and slowing down the LCT-TFT clock to 20MHz.  My LCD brightness still looks okay to me at this slower rate.
Thanks for starting this post that led me toward this solution!