How much mulch you need? Double that estimate!
About 20cm of mulch will pretty much eliminate weeds, as well as doing all sorts of other great things.
Carrots, parsnips and onions are the only things that don't get mulched in my garden. They're a small
enough part of my garden that weeding them's not an issue.
Everywhere else, I pull back a bit of mulch, add
compost, seed/seedling. Voila.
I avoid turning the soil and exposing weed seeds and have very few come up.
Everywhere that's not a garden is a rather healthy 'weed-patch' and I spend nearly as much time admiring those plants as mucking around in the garden itself.
While I encourage weeds outside the beds, they aren't welcome in the garden and the odd one that gets through doesn't last long.