You could strap a band with a strong magnet around your tummy and have an IMU sensor below your mattress. It was a project I started and sampled it at 1 Hz, multisampled with min/max/avg, but I never did anything with the data.

Looking at the real-time stream the breathing was noticeable, at 2Hz it would probably be very useful, if you have the dedication to write the tools to analyze the data.

I was thinking about doing this with a fanny pack where I put the sensor and battery pack in the fanny pack and a strong magnet at the opposite side of the strip in order to measure my breathing frequency during excercising.