The saved are resurrected and supposed to all be with Christ during the 1000 years
You already answered this question with Revelation 20. There's one detail about Revelation 20 though that you apparently have missed.
"... the
souls of them that were beheaded...."
Note these are souls, not bodies. Are those "reigning with Christ in the millennial kingdom" "in soul format" only? Apparently so. The beginning of the "millennial kingdom" did not commence with "the rapture".
and all the wicked are dead until after it.
Note John 6:39-44 and the term "last day". The bodies of the dead are raised on the "last day".
36 - And this is the Father's will which hath sent me, that of all which he hath given me I should lose nothing, but should raise it up again at the
last day.
40 - And this is the will of him that sent me, that every one which seeth the Son, and believeth on him, may have everlasting life: and I will raise him up at the
last day.
44 - No man can come to me, except the Father which hath sent me draw him: and I will raise him up at the
last day.
Now note John 11:24 Martha makes a statement to Jesus about Lazarus. She says "I know he will rise again in the resurrection on the
last day.
John 12:48 "He that rejecteth me, and receiveth not my words, hath one that judgeth him: the word that I have spoken, the same shall judge him in the
last day.
So when is the bodily resurrection? It is on the "last day"; everyone will be raised.
Matthew 25:
31 When the Son of man shall come in his glory, and all the holy angels with him, then shall he sit upon the throne of his glory:
32
And before him shall be gathered all nations: and he shall separate them one from another, as a shepherd divideth his sheep from the goats:
33 And he shall set the sheep on his right hand, but the goats on the left.
34 Then shall the King say unto them on his right hand,
Come, ye blessed of my Father, inherit the kingdom prepared for you from the foundation of the world:
41 Then shall he say also unto them on the left hand,
Depart from me, ye cursed, into everlasting fire, prepared for the devil and his angels:
45 Then shall he answer them, saying, Verily I say unto you, Inasmuch as ye did it not to one of the least of these, ye did it not to me.
46
And these shall go away into everlasting punishment: but the righteous into life eternal.
This is heaven with Christ?
Yes. Jesus said to one of the thieves "Today you will be with me in paradise." Luke 23:48
2 Corinthians 12:2-4 tell us "paradise" is in "the third heaven".
Revelation 2:7 tells us "the tree of life" is in "paradise".
When did the resurrection of the saved take place?
Because of the atonement "in time"; Those that die in Christ, their souls go strait into heaven. (Revelation 6:9 & 20:4 - both use the term "souls". Are people martyred in the millennial kingdom? Apparently yes!)
Rev 20:4 And I saw thrones, and they sat upon them, and judgment was given unto them: and I saw the souls of them that were beheaded for the witness of Jesus, and for the word of God, and which had not worshipped the beast, neither his image, neither had received his mark upon their foreheads, or in their hands; and they lived and reigned with Christ a thousand years.
Rev 20:5 But the rest of the dead lived not again until the thousand years were finished. This is the first resurrection.