John Chrysostom

Now that you may understand what is the difference between statements which are diverse and contradictory, one of the evangelists has stated that Christ carried the cross, another that Simon the Cyrenian carried it: but this causes no contradiction or strife. "And how," you say, "is there no contradiction between the statements that he carried and did not carry?" Because both took place. When they went out of the Praetorium Christ was carrying it: but as they proceeded Simon took it from Him and bore it.

On the Paralytic Let Down Through the Roof ↗

Theophylact of Ohrid

These servants of the devil compelled, as it is said, a certain man to carry His cross; meanwhile, another evangelist says that Jesus went out, bearing the cross Himself (Jn. 19:17). But both things happened: at first He Himself carried the wood of the cross for some time, and when they found another able to carry it, they compelled this latter man, and the cross was borne by him. And why is it also mentioned whose father this man was? …

Commentary on Mark ↗

Gregory the Dialogist

This is well signified by that Simon who, found on the road, carries the Lord's cross under compulsion. For burdens belonging to another are carried under compulsion when something is done through the pursuit of vanity. Who then are designated by Simon, if not the abstinent and arrogant? They indeed afflict the flesh through abstinence, but they do not seek the fruit of abstinence within. …

Forty Gospel Homilies, Homily 32 ↗

Bede

Or, since this Simon is not called a man of Jerusalem, but a Cyrenian, (for Cyrene is a city of Libya,) fitly is he taken to mean the nations of the Gentiles, which were once foreigners and strangers to the covenants, but now by obedience are heirs of God, and joint heirs with Christ. …

Catena Aurea by Aquinas ↗