Why must that thousand years be spent in Heaven by the saints?I have an answer to this, but @ReChoired is more scripturally literate than I so will be able to offer a more comprehensive answer. Suffice to say that your take on the matter is not unique, but does hold some flaws. I would suggest that being a priest and reigning as Kings with Christ has a different meaning than simply ruling, and is more in harmony with judging, as per 1 Corinthians 6:3. There is much to be added to this for our time in heaven over that 1000 years is not spent simply having a good time. The will be a work to do in conjunction with the Lord, that by the end of the millennium all questions will be answered... Why did he/she miss out... What is he doing here... How come she made it?.... Why did You do this?... What happened that caused such in such to be allowed to do so much damage... Etc etc. This will be a high Court case in which all evidence will be submitted and men, such as you and I, will be given full access to scrutinise every detail. By the end, God's sentence upon the wicked will be totally vindicated and justice will not just be done, but seen to be done... There will be no further debate or doubts in the entire universe regarding God's character and the methods He used in dealing with the sin problem and Lucifer's rebellion. That word judge with relation to angels has a connecting sense to ruling as a king and priest.. Judging therefore in a spiritual paradigm and taking along with us our experience as eye witnesses to the drama.
What I see is that God can judge some people before the Thousand Years begins. They know the truth and chose wisely. There is no need to postpone their judgment. The rest are not judged until after the Thousand Years.
Judging? I think the saint never judges to condemn. The saint looks at a person and the situation he's in in order to present him with solutions for his problems. Thus saints do not quarrel with each other or need to go to court with each other. They can talk to each other and come up with solutions. I see the saints reigning as kings as exercising that kind of judgment. I tend to take this parable a bit more "literally" than some others may:
Luke 19:16 Then came the first, saying, Lord, thy pound hath gained ten pounds.
17 And he said unto him, Well, thou good servant: because thou hast been faithful in a very little, have thou authority over ten cities.
18 And the second came, saying, Lord, thy pound hath gained five pounds.
19 And he said likewise to him, Be thou also over five cities.