Why, if Jesus felt Peter was to be his most important disciple upon which the future church would be built, is so little written by him? Instead, so much seems to be built upon the writings of Paul.
If this has been asked and answered before apologies in advance! I did do a search

Peter developed in such a way that he became more steadfast in the face of persecution or threats. In the beginning he still denied being a disciple of Jesus, in a later case he came out and did not deny it. I see a development in that, that he becomes more steadfast as a rock. On such a mentality, a church can be built. Jesus says it in a predictive manner, as if he isn't that rock yet, but has the ability to develop in that direction. That's how I interpret it.

St-Paul was already strong, but as a persecutor. Probably he only needed to change the direction of his aims.

It is widely held that Mark's gospel was actually a recollection of Peter's testimony. Mark was apparently discipled by Peter. Paul had more ground to cover, seeing he travelled so much to bring the gospel to the Gentiles. Hence the more letters to the different churches with his authorship. This seems reasonable to me.

Blessings,

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John