OP's question 1st question;

Col1:15 -18 First Born. Are you saying that the words Supremacy and First Born are synonymous?

No, supremacy and firstborn are not synonymous.

2nd question; "Are all firstborns considered supreme? "

First borns are not always supreme. Look at the example of Esau and Jacob. The inheritance was to go to Esau because he was the first born. That was customary for the first born to get double inheritance, and the blessings. Instead, it was given to the second born.

3rd question; Why didn't Paul just use the word supremacy?

The word supremacy means; : highest in rank or authority the supreme commander especially : in a position of unquestioned authority, dominance.

Even though the first born here has all these qualities, He has been gifted from the Father, who owns all things first. He is showing a love relationship between the Father and the son. Everything comes from the Father first

Who has first given to God, that God should repay him?” For from Him and through Him and to Him are all things. Romans 12:36

So getting back to the last question; "Why didn't Paul just use the word supremacy?"

Perhaps it is because the Father so loves His Son and wants to give him everything. He is just not supreme and a dictator, but is filled with His Father's love that He passes on to His creation.

I think the Father's heart is reflected in the story of Abraham, having everything, but does not a son to inherit his everything. It meant more for Abraham to have his very own son that he could give an inheritance to.

But Abram replied, “O Sovereign LORD, what good are all your blessings when I don’t even have a son? Since you’ve given me no children, Eliezer of Damascus, a servant in my household, will inherit all my wealth. Genesis. 15:2

Abraham left everything he owned to Isaac. Genesis 5:5

My master’s wife Sarah has borne him a son in her old age, and my master has given him everything he owns. Genesis 24:36

Next question;

"The question that arises in Colossians and other scriptures that use the word firstborn is not whether Jesus is supreme, but why did Paul and John use the word firstborn. "

God wants to keep identifying who His Son is by also calling Him first born in everything. His identity reflects who God is and the therefore seen in His Son.

In these last days he has spoken to us by a Son,whom he appointed heir of all things, through whom he also created the ages. He is the reflection of God’s glory and the exact imprint of God’s very being, and he sustains all things by his powerful word. When he had made purification for sins, he sat down at the right hand of the Majesty on high, having become as much superior to angels as the name he has inherited is more excellent than theirs. Hebrews 1:2-4

The Father has entrusted the Firstborn with everything that has come from Him.

The Father wants all to honor His Son and make sure everyone knows who His first born is.

that all may honor the Son according as they honor the Father; he who is not honoring the Son, does not honor the Father who sent Him. John 5:23

There is great significance of being called the first born:

"The laws and customs of all nations show that to be “firstborn” means, not only priority in time, but a certain superiority in privilege and authority." In the Old Testament, the firstborn son was the one who normally received a double inheritance, and was the one who would inherit his father’s role as head of the family." Quoted from Bible . Org questions