and every island did flee away, and mountains were not found,
Revelation 16:20 -
https://www.biblegateway.com/passage?search=Revelation 16:20&version=YLT
Which corresponds to heaven and earth fleeing away and not being found:
And I saw a great white throne, and Him who is sitting upon it, from whose face the earth and the heaven did flee away, and place was not found for them;
Revelation 20:11 -
https://www.biblegateway.com/passage?search=Revelation 20:11&version=YLT
Revelation 16:20 And every island fled away, and the mountains were not found.
21 And there fell upon men a great hail out of heaven, every stone about the weight of a talent: and men blasphemed God because of the plague of the hail; for the plague thereof was exceeding great.
Include verse 21 though before deciding verse 20 fits with Revelation 20:11-15.
Revelation 16:21And there fell upon men a great hail out of heaven, every stone about the weight of a talent: and men blasphemed God because of the plague of the hail; for the plague thereof was exceeding great.
Revelation 20:11 And I saw a great white throne, and him that sat on it, from whose face the earth and the heaven fled away; and there was found no place for them.
12 And I saw the dead, small and great, stand before God; and the books were opened: and another book was opened, which is the book of life: and the dead were judged out of those things which were written in the books, according to their works.
13 And the sea gave up the dead which were in it; and death and hell delivered up the dead which were in them: and they were judged every man according to their works.
14 And death and hell were cast into the lake of fire. This is the second death.
15 And whosoever was not found written in the book of life was cast into the lake of fire.
I don't see Revelation 16:21 fitting with Revelation 20:11-15. And if it can't fit then neither can Revelation 16:20 fit with Revelation 20:11-15.
As to Revelation 6:14 and Revelation 16:20 describing the same events, no disputes there since I tend to agree.