Would cfsetspeed() work on MacOS? Setting input and output peed independently is not necessary. cfsetspeed() is supported in Linux and should also be in MacOS.
Just tested it in Linux and works as expected...

Would cfsetspeed() work on MacOS? Setting input and output peed independently is not necessary. cfsetspeed() is supported in Linux and should also be in MacOS. Just tested it in Linux and works as expected...

Confirm, it works on macOS

Another thing that works is initializing the struct to zero before setting any fields:

memset(&my_termios, 0, sizeof(struct termios));

I guess the root problem here is garbage on the stack being left inside the struct, so either be very sure to set every field or initialize it to 0. Initializing it might be a good idea anyway, uninitialized fields sneak around and cause insane bugs.