Thousands of years are endless: describe a long time. With "boundless".
Forever: forever: forever. Of spiritual or moral eternity.
Endless: Describe a long time ago.
A thousand years: refers to a very long experience. With "ten thousand generations".
It is unique throughout the ages: it has long-term value; Have unique expertise or advantages. Use "unique".
Long-term uniqueness: unique and lasting value; Have unique expertise or advantages that can be passed down through the ages.
Eternal romance: refers to elegant things that have been handed down for a long time.