Why does God choose the tiniest paintbrush to do his detailed work? When Christ went to Golgotha to be crucified, on his way there he chose Simon of Serene, a humble man, and a complete stranger, to carry His cross.

Before that God chose Mary and Joseph to be Son of Man's earthly parents who were just simple people. So why did God choose simple people like them?

Similarly, He chose St. Bro. Andre of Canada to do His impossible work. St. Bro. Andre was a little educated man. Had no wealth or money. He was a doorman. How could he do such huge task? He asked himself a thousand times probably. But he did it.

My question is why does God choose the tiniest paintbrush to do his detailed work?

There are many rich people who are honest and have human elements and attributes. Wouldn't they have done the same task in a more authoritative way?
Your question is fully answered by the Apostle Paul :

Ye see your calling, brethren, how that not many wise men after the flesh, not many mighty, not many noble, are called: But God hath chosen the foolish things of the world to confound the wise; and God hath chosen the weak things of the world to confound the things which are mighty;And base things of the world, and things which are despised, hath God chosen, yea, and things which are not, to bring to nought things that are:
That no flesh should glory in his presence. [I Corinthians 1:26-29, KJV.]

All of the praise and the honour and the glory in justification, redemption and salvatuon - and in all the work and administration of God in connection to it - is due to God alone and not to that which comes of flesh and blood.