Public transportation from Seattle to Sumner is not that great in the direction you'll be commuting. There just aren't that many people doing it. Also, Sumner's public transit routes tend to cluster around park and ride lots, so unless your employer happens to be the route it is likely you'll wind up walking quite a ways.
You can look at the trip planner at
Metro's Online Trip Planner to see whether or not the bus will work for you. I just did a quick entry from downtown Seattle to Sumner, and the trips were all about two hours including a transfer and wait - and that is for 7:30 in the morning. If I left downtown Seattle at 7:30, I wouldn't get to Sumner until 9:15 or 9:30. You could probably make the drive in an hour, which you don't seem to want to do.
You could look at neighborhoods like Beacon Hill or Rainier Beach or places like Tukwila, Renton, Burien, Des Moines. You'd still be about 20 minutes from downtown in the evenings for your events and you'd cut some of the driving time off your commute to Sumner.