‘Then each of them was given a white robe and they were told to wait a little longer, until the number of their fellow servants and brothers who were to be killed as they had been was completed.’ ( Rev 6 : 11 )
The full number of Gentiles have to come in, before Israel will be saved ( Ro 11 : 25 ) and those who didn’t get saved before rapture were saved during the tribulation and that is the number Rev 6 : 11 refers to. Indeed the number of Gentiles who will be saved during the tribulation is know by God and they are the ones being addressed as ‘brethren’.
If we go by your assumption that all of them were not slain, shall the same apply to Rev 13 : 15, which says the beast that came up from the earth ‘ caused all who refused to worship the image to be killed’ ? So the ones who escaped earlier in Rev 6 : 11, will now face the inevitable martyrdom or should we assume again that some of them escaped for 1 Thess 4 : 17 to be fulfilled ? If so, when are they going to be raptured? At the second coming of Jesus ? Why go up when Jesus and all the holy ones are coming down ? ( Rev 19 : 11)
All through the Bible we see that when God judged the wicked, the righteous escaped his wrath. Ex : Noah from the Floods, Lot from Sodom, the Hebrews from the plagues in Egypt, Rahab from the fall of Jericho etc and these narratives unequivocally back up this Scripture :
‘For God did not appoint us to suffer wrath but to receive salvation through our Lord Jesus Christ.’ (1Thess 5 : 9 )
But the strongest argument for a pre-trib rapture is the fact that the ‘man of lawlessness’ cannot be revealed unless the person of the Holy Spirit leaves the earth ( 2Thess 2 : 6 ) and the Holy Spirit will not leave without the Bride of Christ going along with Him. ( Just as Eliezer, the servant of Abraham would not return without taking along with him Rebecca as a bride for Issac, the ‘offspring’ of Abraham, so also the Holy Spirit will not return without taking along with him the Church as a bride for Christ, the ‘seed’ of Abraham.)
Together they wait for the appointed time ! ‘The Spirit and bride say, “Come!” (Rev 22 : 17 )