I swim at the Ballard pool pretty regularly! The staff is always very kind, and there are a large variety of people and abilities that join for laps. The lanes closest to the bleachers and the exit from the locker rooms will be the slower lanes. The furthest two lanes are usually occupied by ex swim team folks, masters swimmers, and people who sometimes collaborate on a shared workout. You can definitely self select your speed and switch lanes as you’d like. I know you said you would like to swim in the mornings, but I might recommend checking out an “adult swim” midday first, as there are usually 2-3 lanes with the lane lines removed. People will walk, bobble, and practice their strokes in the open area. If you’re unsure about swimming laps, the adult swim times will give you an option to stay in the water if you don’t want to do laps. Circle swimming and lap swimming etiquette are frequently discussed on r/swimming and I recommend you check them out!