That brings us full circle to the debate over whether Gen 1.1 is meant to be understood as a summary/title or initial act of creation. But difficulties are encountered either way, regardless. We're left with two basic options, neither of which is appealing.
Option 1: If Gen 1.1 is God's first act of creation ("created the heavens and the earth"), then why did God 'create' and formless and void state of chaos that is antithetical to everything He does during the six day creation week? Ex 20.18 also limits creation to the six days (which suggests Gen 1.1 is a title/summary, not a first act). We also have the problem that God doesn't create an inhabitable earth until Day 3, nor the sky/heavens until Day 2.
Option 2: However, if Gen 1.1 is a title/summary statement and not an initial act of creation, that solves some of the problems above, but then creates the problem of pre-existent matter prior to God's start of creation.
Either way, v.2 presents a problem. I don't claim to have a solution. I agree with you that the Gap Theory doesn't solve the problem, and requires reading a whole back story in between v1 and v2, which is completely conjectural.