How do you conclude that Prophet Muhammad SAW was the imam of all prophets?

We know from Quran verse 2:253 that God allowed certain differences among his prophets, however, those differences are from God’s perspective according to His infinite wisdom. As believers we have been asked to make no distinction among any of His apostles.

Quran 2:136

Translation by Asad

Say: “We believe in God, and in that which has been bestowed from on high upon us, and that which has been bestowed upon Abraham and Ishmael and Isaac and Jacob and, their descendants, and that which has been vouchsafed to Moses and Jesus; and that which has been vouchsafed to all the [other] prophets by their Sustainer: we make no distinction between any of them. And it is unto Him that we surrender ourselves.”

Quran 2:285

Translation by Asad

THE APOSTLE, and the believers with him, believe in what has been bestowed upon him from on high by his Sustainer: they all believe in God, and His angels, and His revelations, and His apostles, making no distinction between any of His apostles; and they say: We have heard, and we pay heed. Grant us Thy forgiveness, O our Sustainer, for with Thee is all journeys’ end!

Also please note that God has given the status of leadership (imam) of all mankind to Prophet Ibrahim SAW.

Quran 2:124

Translation by Asad

And [remember this:] when his Sustainer tried Abraham by [His] commandments and the latter fulfilled them, He said: “Behold, I shall make thee a leader of men. “Abraham asked: “And [wilt Thou make leaders] of my offspring as well? “[God] answered: “My covenant does not embrace the evildoers.”