I exclusively breastfed until LO was just over 11 months (we did BLW starting at 6 months but she never had formula or cows milk). I started mixing in about an ounce of cows milk to her expressed feeds at daycare around the 11 month mark just to get her accustomed to it. We completely weaned from breast milk at 13 months.
I get how you feel with the expressing at work, it's such a drag!! I wanted to quit so many times, but forced myself to continue. I wanted to make it to the year mark, and I'm glad I was able to do it. I was lucky enough to have enough in the freezer so that I was able to stop pumping at 11 months. She still nursed first thing in the morning and last thing at night, but everything in between was from the freezer with a little cows milk mixed in.