The chief/foremost sinner in context still leads to the same understanding.  God did not use the word, "first" there for Paul being the chief sinner.

He's the number 1 sinner.

He was a blasphemer, persecutor, and insolent/injurious.

Breathing murder, wreaking havoc on the church of God, and hunting down the brethren.

Thank the Lord He interrupted Paul on the road to Damascus (looking to further persecute Christ and His disciples, btw).

The Lord showed forth ALL LONGSUFFERING in saving Paul.

The Lord's grace and peace be with you, sister.