You might consider a different view regarding Jesus' reign:
If you read Rev 20:4 carefully, it does not discuss the length of Jesus' kingship. Rather, it specifies how long his disciples would "reign" with him.
The NT describes Jesus as ruling "until [God] has put all enemies under [Jesus'] feet." (1Co 15:25; Heb 10:12-13; Ps 110:1) For an analysis of when Jesus began ruling, based on what the NT indicates, see this post. And for an analysis of the Greek word parousia (translated as "presence" in the NWT), and that the word is never used in the NT in reference to WHEN Jesus is crowned or given ruling authority, see this post. (Both links are off-site.)
But back to Rev 20:4. See the reasoning used in this post (also off-site) regarding who is being described in Rev 20:4.
Incidentally, I agree with you on the New Covenant being intended for all Jesus' disciples.