The RP2040 is a great MCU for playing with graphics as it can bit bang VGA and DVI/HDMI. There's some info on the DVI here: https://github.com/Wren6991/PicoDVI
You can do something similar on STM32 parts that have an LCD controller, which can be abused to drive a VGA DAC or a DVI encoder chip. The LCD controller at least is fully documented, but many of their parts pair that with a small GPU, which would be an advantage over the GPU-less RP2040... if there were any public documentation at all for the GPU :(
I used "composite" (actually monochrome) video output software someone wrote on the RP2040 for an optional feature on the PhobGCC custom gamecube controller motherboard to allow easy calibration, configuration, and high-frequency input recording and graphing.