This is how I understand it. God is the ruler of the universe. This does not change. He has handed judgment and rulership of the earth and its people over to His son. He will govern for a thousand years. After destroying death, he will hand the kingdom back to his Father.
Time indefinite vs. Forever: Hebr. ʿôlām possibly from ʿālam “to hide,” Gr. aiōn. Here the context should be one’s guide. Most scholars translate ʿôlām as “long time,” “farthest, remotest time” or “perpetuity.” From the perspective of the writer/speaker it can look forward, towards the (distant) future or backwards, bygone times (long ago). E.g., God’s kingship is forever (Rev 15:3; cf. Ps. 10:16a; 29:10b; 146:10). John hears what seems to be the voice of a great multitude, crying out, “Hallelujah! For the Lord our God the Almighty reigns” (Rev. 19:6 ESV, NET; cf. Jer. 10:7, 10; Zech. 14:9, 17; 1 Tim. 1:17). However, Jesus, as Messianic king, will be ruling a thousand years (Ps. 45:6, 17; 1 Cor. 15:24-28; Rev. 20:4).