as for thinking the Lord doesn't punish the flesh for transgressions after being saved, that's just not biblical.
It's not what happens either.
The Lord corrects us when we sin, and convicts us through his Spirit, yes - but that's not the same as being punished "in the flesh"; receiving some kind of illness.
What illness did Peter receive for denying Jesus?
What illness did he receive for eating with Gentiles and then stopping, leading to an argument with Paul and the accusation that he was a hypocrite, Galatians 2:11-13?
What illnesses did the 12 receive when they failed to trust Jesus, or believe his words, e.g Mark 8:17, Luke 9:46?
There are, and have been, Christians who've made the headlines because they've sinned big time - had an affair, been found guilty of fraud or whatever. There has been no report to say that they've been struck down by illness as a punishment.
Jesus took the punishment for our sin so that we wouldn't have to. If God was going to send illness as punishment for sin, or to correct a believer, there would be no point in Jesus having died. God could say to each of us, "you've sinned: the wages of sin is death; I'm going to save your soul but kill your body because of your sin". There'd be no Christians in the world - everyone who accepted Jesus and was born again would then die because of their sin, but their souls would be safe with God. There would then be no one to serve God, carry out the Great Commission and so on.
Clearly, it doesn't happen like that.
Why every son? Because we all sin, even after being saved.
Yes, but we aren't all punished with illness or other problems.
If we were, every Christian in the world would constantly be in poor health, or just recovering. In fact, no one would recover from one illness before being hit with another. No one has perfect thoughts/motives/desires 24/7.
Paul is trying to reassure people that if bad things are happening in their life, it's not because God doesn't love them, or God doesn't care about them, or they're not truly saved. Paul is telling them quite the opposite, that because they're dealing with these afflictions and negative things happening in their life, that God is correcting them, disciplining them for sin.
That may have been the case
then.
The book of Hebrews was written to a church which contained Gentile, and Jewish, Christians, and they were facing a time of persecution, or would be in the future. It seems that some of the Christians were thinking of trying to escape that persecution by going back to the synagogues and declaring that they were Jews. The trouble was, though, that in order to have been accepted by the synagogues, they would have had to publicly declare that they had made a mistake and that Jesus was NOT the Messiah. So when they were persecuted, they could save themselves from death - by renouncing their faith.
Paul is encouraging them not to do this.
This is why he says that if someone turns away, it is impossible to turn back again; they couldn't renounce their faith in Jesus, become Jewish until persecution had passed, become Christians again, and renounce their faith again the next time there was persecution.
They may have been kept physically safe by denying Jesus, but they would have lost far more spiritually.
This is not at all the same as saying that every time we get a cold, tummy bug, cancer or break a bone, that is punishment from God for having sinned.
That disciplining over bad behavior can be a comfort because hey, at least you know God is with you. But that said, it should be a correcting action, it should make you examine your deeds and think "am I walking contrary to God, am I EARNING stripes?" if so, well, you don't want to earn more stripes do you?
What about those who have pretty healthy and problem free lives and yet ARE disobeying God - or those who are not disobeying God and yet are in wheelchairs?
As I said, why isn't every single Christian in the world in a permanent state of ill health? You yourself have admitted that we all sin as Christians.