I just remembered something that makes a difference; Chase recently hasn't been letting downgrading the Sapphires during the first year, so in the calculations we have to consider all spend for the CSP after 4k as 1ppd if getting CSP SUB.
That would make:
CSP - 24K spend: 55K/4K(50K SUB+5K AU)+20K/20K= 75K
CFU - 24K spend: 72k (@3ppd)+2.5k (AU) =74.5k
So it would take about 25k non-bonus spend during the first year to come out ahead with the CFU.