None of the Four Evangelists says Jesus had brothers by Mary, i.e. sons other than Jesus, who were borne by Mary. That is the point in dispute.
Nobody--- as you know --- disputes the fact that Jesus had brethren in the sense of near kinsmen.
Not when read in context.
When one reads the Four Evangelists, to use your term, one understands they knew Jesus had brothers and these were from Mary.
Does the NT record the date and time Joseph and Mary had relations? No.
But it does indicate in Matthew 1:25 they did have relations, or knew each other, after Jesus was born.
When the family is mentioned in the Gospels, the Jews seem to have no problem identifying the brothers and sisters being in His family.
Why didn't the writers provide more on this? Don't know other than the primary focus of the NT is on Christ. His brothers and sisters are not the focus nor are Joseph and Mary.
If you introduced someone as your brother or sister the vast majority of people would deduce they were from your mom and dad.
They wouldn't go into some long drawn out debate wondering if they were from another marriage, did your parents "know" each other, etc.
Context is your friend in reading the Word.
None of them say what you're claiming either.
However, we have the rest of Scripture to support our claim.
You don't.
You have to appeal to extra-Biblical nonsense and writings to support yours.