Logging off at nine and meaning it gives everyone else permission to do the same.

Whoever sets the pace in a group is rarely the person stating what is expected. It is the one whose behaviour everybody is quietly matching, and a message sent late reads as a standard no matter what the words around it claim. The people below you feel it most, because the cost of guessing wrong is highest for them.

Pick a time and hold it visibly for two weeks, including on the nights when it is inconvenient. If you must work late, schedule the message to arrive in the morning. What you do is the policy.