I dont think it is a problem with saying "hey" but when you say "hey" by itself. All the time I will say something like "hey" and then a followup message with a short question or asking how they are doing, etc etc.
But for sure it is annoying when someone sends me a message that says "Hi" and then says nothing else until I respond
I’d say it would even better if you just put a couple of newlines after “hey” and put your question in the same message. One less notification would be appreciated by your coworkers.
The person you're sending a message to doesn't need to sit there and watch you type out your actual message.
Just put "Hey" at the beginning your message. Don't split your message up into a separate message for "Hey" and a separate message for your real message.