9 Itinerary - I'm stuck on the picking a final location.

We were planning on using trains.

Since it looks like you only have one full day in the BO and stated more interested in outdoor activities, you may just want to stay there. There’s quite a bit to do and see in that area.

If you just spend 4 nights in the Berner Oberland instead of 2, you will probably not regret it.
If you want to keep moving, I would spend 3 nights there, then 2 in Bern, and just 1 in Zürich. Zürich is nice to explore for a few hours, but I think it does not warrant 2 nights in a short Swiss trip.

Some good things in the BO don't open until June 8th, so I'd recommend being there after that for things like the Allmendhubel & Schynigge Platte. By doing your suggested itinerary you're not allowing enough time in each place to really enjoy them and do even a portion of the activities in the area, as well as leaving no buffer for poor weather, which you will have. 1 full day in a place is not enough and with 9 days I'd pick 2 locations. Or 2 locations + an afternoon/evening in Zurich if your flight leaves before noon.

June 4 - Arrive in Zurich, train to Luzern
June 5 - Luzern
June 6 - Luzern
June 7 - Luzern > BO (Wengen or Murren)
June 8 - BO
June 9 - BO
June 10 - BO
June 11 - BO > Zurich
June 12 - Fly out of Zurich

I like wanderweg's suggestion. No one is ever really ready to leave the BO. Safe travels!

You will not want to leave the Berner Oberland. Stay as long as you can.

I also like wanderweg's suggested itinerary. You will not want to leave the Berner Oberland and there is enough there to keep you busy for months.