In which moment or album do you think the band defined its style?

I'd say Hang Time.

Also, it's somewhat surprising Made to Be Broken came out before While You Were Out, yet the latter seemed to follow more in the footsteps of their very first record: punkish, all over the place, lacking cohesion, while the former one offered some new elements and a more stylized sound.

I would have to agree with Hang Time, though I think their sound continued to evolve. I think their acoustic "Runaway Train" sound appeared around GDU & they tried new sounds on COF. But I think they started sounding like SA around Hang Time.

I think every album, more or less, represents the style they were going for at the time. They've always kept evolving, at least to some degree.