I had same issue, I would swipe all over the screen moving the auto focus box erratically, and eventually zoom slider would appear.
Here's the trick, just BEGIN a two finger swipe (two fingers swiping inwards towards each other, since camera defaults to zoomed out fully) and that will bring out the slider.
Then, use slider, or keep two finger swiping.
I definitely wish zoom slider was always visible. Quite often I'm holding phone in a way that it's much easier and practical to use slider. I can't hold phone and swipe with two fingers with one hand, simultaneously! But can with the slider.