1. The bodily resurrection of the martyrs, who are of the figurative 144,000 under the OC., after Jesus' mortal death, while on the cross. Please see KJV Mat. 27:50-54.
2. The figurative 144,000 remaining of the OC. permanently receive the "seal" of the HS after Jesus' death and resurrection. Please see Rev. 6:9-11.
3. Both Jew and Gentile, of faith in Jesus since Pentecost, are of the Great Multitude under the NC. See Rom. 10-12.
Of course David didn't bodily ascend to heaven yet, he wasn't a martyr of the "many" that were bodily resurrected right after Jesus death on the cross. Surely you don't believe that the many who were bodily resurrected then died a 2nd mortal death, do you??
David is of the figurative 144,000, who are "remembered" by God, "under the altar" of the OC. Therefore David, having received a "white robe", is in Paradise, along with the repentant thief, and is asleep resting in Jesus, waiting for Jesus' return.
Again, the "many" (not all), that were bodily resurrected by the Holy Spirit after Jesus death, are only those who were martyred of the 144,000 under the OC.
However, since Jesus' death and resurrection, ALL who were of faith in Israel, who weren't martyred, including David, are of those that God remembered, being the figurative 144,000 out of the 12 tribes of OC Israel.
After Jesus suffered mortal death and gave up the ghost, He went by His Holy Spirit and visited those who had died naturally under the OC., whom God had "remembered". Malachi 3:16-17; 1 Peter 3:18-20.
They each were then given a "white robe" [sealed], which is symbolic of the permanent Gift of the Holy Spirit of God (Rev. 6:9-11), but were told that they must rest yet awhile longer, until their fellow servants and their brethren should be killed as they were, being those who are under the NC.
When Jesus returns in all his Glory, in flaming fire, God shall bring with Him all who sleep in Jesus, being those who had died in faith of His coming in the flesh, both those of the OC and the NC.
1 Thes. 4:14.