Don't measure the seat tube. Measure the top tube/effective top tube. That's the measurement that matters.

Ok so I looked up the size chart. A 59cm frame has a 57.5cm effective top tube length. Not bad, but still on the shortish side.

Based on your leg angle, I don't think lowering the saddle is the right thing to do, you have a good bend. It looks like your saddle is already pushed back a bit, try going a bit further + shallower drops (compact road drops like: 3T Ergonovas) and it may help.

Lowering the tip f your saddle a bit will also help to flatten out your back (as someone above mentioned).