In the audible frequencies for the example I provided, ESR is insignificant.
in your opinion it is insignificant.
You acknowledge that R is not a factor in the relevant equationin your opinion this is the only relevant equation
your choice to ignore ESR therefore also ignores the fact that ESR is frequency-dependent, temperature-dependent, and changes as components age with some capacitors.
I never said your formula for Xc is incorrect, but filter calculations include R, not just Xc, yet all you quote is the formula for Xc while ignoring the formula for cutoff frequency of an RC filter.
I’m saying since you are deciding which factors and which equations are applicable while ignoring other factors and equations your conclusions seem correct ... to you.