I'm trying to follow what you are saying, and think I understand.
I'll try to cover everything, just in case I might be missing something.
So, Peter, James and John -
some of those standing there, did not taste death, until they saw the kingdom of God coming with power.
They saw the kingdom of God coming with power a few days after Jesus' promise.
The vision of the transfiguration was the kingdom of God coming with power.
How do we know?
All three Gospel accounts refer to what happened a few days after Jesus' words, linking the transfiguration vision to the kingdom of God coming with power.
We need not assume Jesus was saying anything else, like maybe a city coming down out of heaven, or a white horse on the clouds.
Seeing the kingdom come, can be as simple as seeing a representation of the kingdom.
For example, in the vision, Jesus' face shone like the sun, and his clothes became as white as the light.
Mark says, His clothes became dazzling white, whiter than anyone in the world could bleach them.
Luke says, As he was praying, the appearance of his face changed, and his clothes became as bright as a flash of lightning.
This is glory, we agree.
Matthew 24:30 reads, ...
They will see the Son of Man coming on the clouds of heaven with power and great glory.
A cloud did cover them, and God's voice was heard. Can we see Jesus' power. It's not a physical element.
So, Matthew 24:30, actually compares to the transfiguration... Only, the latter, is a vision, while the former is the actual occurrence
We can say then, that the three disciples on that mountain, saw the kingdom of God coming with power, because they saw the glorified king - Jesus Christ, in all his power.
We also have confirmation from Peter himself.
Recall what Peter later said.
2 Peter 1:16-19
16 For we did not follow cleverly devised fables when
we made known to you the power and coming of our Lord Jesus Christ, but we were eyewitnesses of His majesty.
17 For He received honor and glory from God the Father when the voice came to Him from the Majestic Glory, saying, “This is My beloved Son, in whom I am well pleased.”
18 And we ourselves heard this voice from heaven when we were with Him on the holy mountain.
19 We also have the word of the prophets as confirmed beyond doubt. And you will do well to pay attention to it, as to a lamp shining in a dark place, until the day dawns and the morning star rises in your hearts.
Peter connects the transfiguration vision, to the power and coming of our Lord Jesus Christ.
So, we have scriptural evidence, both from the context, and verified eyewitness testimony.
Just then there appeared in glorious splendor, before them Moses and Elijah, talking with Jesus
Peter said to Jesus, ‘Rabbi, it is good for us to be here. Let us put up three shelters – one for you, one for Moses and one for Elijah.’ (He did not know what to say, they were so frightened.)
They did not realize what they were seeing was a vision, but Jesus made that clear. Matthew 17:9
What about Moses and Elijah talking with Jesus?
Luke 9:31, 32
31 They spoke about his departure,[
a] which he was about to bring to fulfilment at Jerusalem.
32 Peter and his companions were very sleepy, but when they became fully awake, they saw his glory and the two men standing with him.
What they represent, is left to the discernment of those whom Jesus has appointed as a light to the nations.
However, Peter can help. In the same verses 2 Peter 1:16-19, verse 19 says in part...
We also have the word of the prophets as confirmed beyond doubt.
Two great prophets were in the vision.
Peter draws attention to the word of the prophets as confirmation beyond any doubt.
Whether that pertains to what Peter heard these prophets say on that mountain, or past words of these prophets, we can connect the prophets Elijah and Moses, to Peter's words, which could/might shed light on why they were in the vision.
I see that connection, but perhaps someone else sees something else.
I don't think we can be dogmatic about the significance of Moses and Elijah conversing with Jesus.
A vision of that event would also signify that. Would you agree?