An official annulment from the Church Tribunal is NOT required if the conscientious Catholic, in the “internal form” (your own conscience) is convinced that the marriage was not valid.
I’m getting married (outside the church) before receiving my expected annulment.
The key terminology in my mind is that the Tribunal “declares the marriage to have been a nullity” (my wording). That means that it already is a nullity (never was a marriage), but just must be formally declared so.
That said, I still understand that while a marriage outside the Church might not be a sin (adultery), the Declaration is needed for a marriage within the church.
Look up the “Internal Forum” — there were some efforts by the “law and order” types, within the Church, to require that some form of official recognition of an “internal forum” decision had been made.
The Vatican REJECTED that move, saying that the “forum” would no longer be “internal” if formal regulations were required.