You think that Revelation 20:6 "Blessed and holy are those who have part in the first resurrection..." refers only to Revelation 20:4 "I saw the souls of those who had been beheaded...", which is why you highlighted it in that way.
Let me highlight it another way:
I saw thrones on which were seated those who had been given authority to judge. And I saw the souls of those who had been beheaded because of their testimony for Jesus and because of the word of God.
Taken this way, the judges and the martyrs are not necessarily the same group.
I have nothing left to say.