The cat-smell-apartment page has twelve fixes on it at this point, which makes a useful menu but a bad starting point. So here's the actual ranking, after living with all of it for a while: two things did most of the work.
The moisture absorber tub is the single biggest difference-maker. It sits flat on the floor right next to the litter box — somewhere a hanging bag can't go — and pulls ambient moisture out of the air in that exact spot, which turns out to matter more for the lingering smell between cleanings than any spray or enzymatic cleaner does.
The other one is less obvious: the litter box itself. A stainless steel box doesn't hold odor in surface scratches the way plastic does after a year of scooping, so the box stops being a smell source on its own instead of just containing one. If you've deep-cleaned a plastic box and it still smells faintly even empty, the box itself is probably why.
Both were already on the full fix list — this is really just flagging which two to do first if you're only going to do two.