You shouldn't manually add /??/bin to the PATH in the MSYS2 terminal.
It happens automatically (along with other changes to environment variables) when you open the terminal for the environment you want to use.
There should be different shortcuts for different environments, the one you're looking for should be named along the lines of ... MinGW 64-bit.
The shortcuts point to executables in C:\msys64, which you can run directly (this one is mingw64.exe).