Consider this...
Revelation 20:6,before the next age...
Rev.20:6 is referring to the resurrection of the great tribulation saints and takes place after Christ returns as revealed in Rev.19:11-21 and at the beginning of the millennial period i.e. the next age.
Revelation 19:20,,,before the next age...
The above takes place after God's wrath and when Christ returns to the earth to end this age and establish the millennial period. As he returns he has the beast and the false prophet captured and thrown alive into the lake of fire. It is also when all of those kings, their armies and generals, etc., will be killed with that double-edged sword that proceeds from the Lord's mouth which is figurative of the word of God. Christ's return to the earth ends the age and begins the millennial age.
Revelation 20:3,,,,after the next age...
Just curious, you identified Rev.20:6 as taking place before the next age, but you have Rev.20:3 as being after the next age?
Rev.20:3 is when Satan and his angels are bound in the Abyss, which takes place just as Christ returns to the earth to end the age and establish the millennial age and after the beast and the false prophet are thrown into the lake of fire.
Revelation 20:12-15,,,after the next age...
Rev.20:11-15 describes the great white throne judgment, which takes place at the end of the millennial age. This is where all of the unrighteous dead throughout all of history will be resurrected and be held accountable for all of their sins. Though people will have already been in punishment in Sheol/Hades, think of the great white throne as their official judgment
All of the scripture, except for Rev.20:11-15, takes place at the end of the age and the beginning of the millennial age