By the way @HyperspaceMadness, here is the log of the crash of Flycast with one of your presets, unfortunately it does not seem to provide much info :
[INFO] [CORE]: Using content: G:\Emulation\Dreamcast\ISOS\Crazy Taxi\Crazy Taxi (USA).gdi.
[INFO] [Core]: Content ran for a total of: 00 hours, 00 minutes, 00 seconds.
[INFO] [Core]: Unloading core…
[INFO] [Core]: Unloading core symbols…
[INFO] [Video]: Does not have enough samples for monitor refresh rate estimation. Requires to run for at least 4096 frames.
[INFO] RetroArch 1.9.4 (Git 6772b89)
[INFO] === Build =======================================
[INFO] CPU Model Name: AMD Ryzen 7 5800X 8-Core Processor
[INFO] Capacités: MMX MMXEXT SSE SSE2 SSE3 SSSE3 SSE4 SSE4.2 AES AVX AVX2
[INFO] Built: May 31 2021
[INFO] Version: 1.9.4
[INFO] Git: 6772b89
[INFO] =================================================
[INFO] [Input]: Found input driver: “dinput”.
[INFO] [Core]: Loading dynamic libretro core from: “G:\Emulation\Retroarch\cores\flycast_libretro.dll”
[INFO] [Overrides]: Core-specific overrides found at “G:\Emulation\Retroarch\config\Flycast\Flycast.cfg”.
[INFO] [Overrides]: No content-dir-specific overrides found at “G:\Emulation\Retroarch\config\Flycast\Crazy Taxi.cfg”.
[INFO] [Overrides]: No game-specific overrides found at “G:\Emulation\Retroarch\config\Flycast\Crazy Taxi (USA).cfg”.
[INFO] [Config]: Appending config “G:\Emulation\Retroarch\config\Flycast\Flycast.cfg”.
EDIT : Found something : the shader crashes because I use a higher resolution than my monitor resolution in Flycast (I use a 4K resolution when my monitor is only 1440p, I use this for downscaling). If I limit Flycast’s internal resolution to 1920x1440, your shaders work.