"2019 to 2022 total contributed 4X$6000=24000"

There is no accumulated total if, every year, they contribute and immediately convert; then every year the FMV (and basis) at the end of the year is $0. That means the starting basis of the account for any tax year is $0, as well. The contribution + that $0 is the total basis for that year, but ends at $0. And the amount converted is that same amount contributed, and since it is all basis (no time delay for any earnings), there is nothing taxable. That's how backdoor works.