I haven't seen any biblical support for the claim that there can be no repentance, forgiveness, or salvation after death.
There are two deaths, the first death is death of sanity, the second is death of the body. And there are two resurrections, the first from false beliefs and degrading practices and the second from this mortal coil. So, right, there is a chance to repent after the first death. Those who fail to do so while they still have a body, game over, they are judged according to their words and deeds, their chance passed, their time in time is over, physical life ended, its too late to repent.
So you are partly right. Jesus preached to the dead. One was taken, the other was left behind.
The second death (of the body) has no power over those who take part in the first resurrection.
On the contrary, I believe the biblical evidence supports the doctrine that ALL will come to repentance and will serve Jesus and then forever in God's kingdom.
"Many will be purified and refined, making their robes shining white,
but the wicked will continue in wickedness and none of them will understand, but those who are wise will understand."
"Many who sleep in the dust of the earth will awaken: some to everlasting life, others to shame of
eternal abhorrence."
"The Son of Man will send out his angels, and they will weed out of his kingdom
everything that causes sin and all who do evil. They will throw them into the blazing furnace, where there will be weeping and gnashing of teeth. Then the righteous will shine like the sun in the kingdom of their Father. Whoever has ears to hear, let them hear."
“Then he will say to those on his left, ‘Depart from me, you who are cursed, into the
eternal fire
prepared for the devil and his angels. For I was hungry and you gave me nothing to eat, I was thirsty and you gave me nothing to drink, I was a stranger and you did not invite me in, I needed clothes and you did not clothe me, I was sick and in prison and you did not look after me. They also will answer, ‘Lord, when did we see you hungry or thirsty or a stranger or needing clothes or sick or in prison, and did not help you?’
He will reply, 'Truly I tell you, whatever you did not do for even the least of my brothers, you did not do for me.' Then they will go away to
eternal torment, but the righteous to eternal life.