There are numerous interpretations regarding what Ellen White meant by her writings are a lesser light.
The expression lesser light comes from the book of genesis.
Genesis 1:16 KJV
And God made two great lights; the greater light to rule the day, and the lesser light to rule the night: he made the stars also..
Moses uses the word greater light instead of sun and lesser light instead of moon.
The sun has its own light. Moon doesn’t have its own light but reflects the light of son.
One school of thought is the writings of the prophets are a lesser light leading to the greater light Jesus.
KJV 2 Peter 1
19 We have also a more sure word of prophecy; whereunto ye do well that ye take heed, as unto a light that shineth in a dark place, until the day dawn, and the day star arise in your hearts:
John the Baptist referring to Jesus says Jesus is greater than himself a prophet. Matthew 3:11 KJV
I indeed baptize you with water unto repentance: but he that cometh after me is mightier (greater) than I, whose shoes I am not worthy to bear: he shall baptize you with the Holy Ghost, and with fire:
In revelation 12 we see the church clothed with the righteousness of Christ (sun) and standing on the writings of the prophets(moon).
Revelation 12:1 KJV
And there appeared a great wonder a in heaven; a woman clothed with the sun, and the moon under her feet, and upon her head a crown of twelve stars:
The psalmist calls writings of a prophets as a light and lamp unto his feet.
Please note when we walk in the presence of sun then there is no need for lamp. Lamp or moon is required in the absence of sun. All the writings of prophets are like moon guiding us until we be with God forever.
Psalm 119:105 KJV
Thy word is a lamp unto my feet, and a light unto my path.