Decks 7-9 are all really nice and are easy to get anywhere up or down from there. If you want truly quiet, I would try to book a room that is not connecting. You can definitely hear families at times with connecting doors depending.
The fitness center is on deck 5 forward.
Adult hot tub on deck 13 aft.
Rainforest Spa hot tubs are with the spa, deck 5 forward with an additional charge (per day, or length of cruise stay pass).
So really, you'll be all over the ship depending on where you want to go. Midship is always popular, but I wouldn't hesitate to book aft or forward. We like midship but also forward as well.
We've done a couple's trip, family trips, and we like the Wish, even though we also love the classic ships. The best time to enjoy the ship's quieter times is during first dinner time. Pools, hot tubs, open decks are quiet and usually sunset time for beautiful views.
Book Palo or Enchante for a nice dinner experience. (Additional, not included restaurant experience) You can book ahead of time or book on the ship when you first get on board, there is a time to adjust dining experiences. You can add a reservation, if available, on a night that you would like once you get your dining rotation onboard. If you are going to skip a restaurant, Marvel or Arendelle would probably be ones to choose, they are dinner "shows" that tend to be loud and somewhat overstimulating. (Not that they're not fun, but 1923 is a nice atmosphere so don't skip that one.)
Let me know if you have any other questions! We just got off the Wish last week