The whole "cloaked cargo ships are undetectable" is pure bullshit. Cloaked cargo ships were regularly detected in SG-1, Earth with Ancient and Asgard tech should have no problem doing the same.
It's not bullshit, it makes perfect sense given what we've seen. To my knowledge cloaked cargo ships have only ever been detected twice in franchise history. Once by Anubis with with Ancient technology, and again by Zipacna when he'd already detected it coming out of hyperspace and had a fleet above the planet, and even then he only found it by scanning for atmospheric disturbances - not something practical on Earth.
I really don't see why Zipacna's method can't be used in any circumstance.
But regardless, there's enough Ancient and Asgard technology on Earth and on its starships that Goa'uld cloaking technology should effectively be obsolete.