It’s possible to do it all with a track pad. On my desk, I use two finger scrolling on the trackpad (left hand) with my Wacom tablet pen (right hand) to get that middle button scroll wheel function of a mouse. Away from my desk, the track pad is usually all I have.
Comparing the two right now, I have to say, SU Free on the web seems to zoom a lot faster than SU Pro on my desktop, so it’s a bit touchy. I’m guessing that may be what you’re describing. I’m not sure if you can adjust that with a control panel setting somewhere.
Also, it matters where the cursor is when you zoom. Play around with it and you’ll see. If you get lost, zoom extents (last icon under obit) on the bottom in SU Free brings everything off screen back into view.