Looking at the NT evidence of the word “firstborn”:
In Matt 1.25 and Luke 2.7 Jesus is described as Mary’s firstborn son.
In Romans 8.29 Jesus described as the “firstborn among many brothers” in the new creation.
In Col 1.18 Jesus is called the “firstborn from the dead”.
So you can see why the straightforward reading of Col 1.15 is that Jesus truly is “the firstborn of all creation”. Origen, who described Jesus as a creature and the oldest creation, and other pre-Nicene Christians understood the phrase in its straight forward sense. Even in the fourth century dispute with Arius the tendency of Trinitarians was to argue that Col 1.15 “the firstborn of all creation” doesn’t refer to the original creation rather than attempting to deny the meaning of the word “firstborn”.