ZW1T Lite cydia tweak brings zoom With 1 Thumb for Safari only.
Double tap but tap different place, and depending on the direction from the 1st tap to the 2nd tap
Double Tap Up to zoom to Fit Item Width
Double Tap Down to zoom to Fit Item Height
Double Tap Left to zoom to Fit 1 Page/2 Width
Double Tap Right to zoom to Fit 1 Page/2 Width
Double Tap Up Left to zoom to Normal Size
Double Tap Up Right to zoom to Normal Size
Double Tap Down Left to Undo Last Zoom
Double Tap Down Right to Undo Last Zoom
Zooming actions of above gestures can be customized in Settings.
Look forward to full version of ZW1T with more practical gestures and functions that work in many more Apps, not only Safari.