In Psalm 89:19 David is called Gibbor, Mighty. Other human beings like the men of David in 2 Sam 23.8 and kings are called mighty as well (ele gibborim = Ezek. 32.21; cp. Isa. 9.6).

In Ps 89:25 the right hand of David is synonymous with the right hand of God (cp. Ex 15.6; Ps 118.16 = godlike King, Ps 72.2, 6, 17a).

The following verses show the similarities between the human king and God:

Of David, Psalm 72:2 HCSB
“He will judge Your people with righteousness and Your afflicted ones with justice.”
Of God, Psalm 98:9b WEB
“He will judge the world with righteousness, and the peoples with equity [i.e., justice].”

Of David, Psalm 72:6 NCV
“Let him be like rain on the grass, like showers that water the earth.”
Of God, Hosea 6:3b BSB
“He will come to us like the rain, like the spring showers that water the earth.”

Of David, Psalm 72:17 HCSB
“May his name endure forever; as long as the sun shines, may his fame increase. May all nations be blessed by him and call him blessed.”
Of God, Malachi 1:11a NIV
“My name will be great among the nations, from where the sun rises to where it sets.”

Furthermore, the Davidic king is called the firstborn, most high in Ps 89:26-37. In other words, he will be the firstborn in rank not time. That’s because the Messiah would be the uniquely procreated (begotten) Son of God (Ps 2.7-12; cp. Rom. 8:29; Col. 1:15; Rev. 1:5).

The Pulpit Commentary literally reads the Hebrew here as the most high above the kings of the earth; cp. God over the kings of the earth, the Most High to his angelic ministers.

As a result, in Ps 89.29, 35-37 the throne of David is eternal just like the Kingdom and its rulers, cp. Ps 78.68-70.

Word Biblical Commentary
“Thus the most high status of the king is the counterpart of that of Yahweh in the heavenly realm. The king does on earth what God does in heaven.”
[cp. Mat 16.19; 18.18 whatever you bind on earth…]
The cosmic ruling power of God is invested, in considerable measure at least, in the Davidic king.”
[Cp. Ps 45, your throne O god; Isa 51.16, God’s servant/agent will establish heaven/earth, cp. Heb. 1.10.]