Clever, but too much work. You should always have your shop vac on the job anyway. Holding a running shopvac hose under the location you're sanding or cutting will capture the dust that falls and draw-in the airborne dust the envelop will never catch.
This seems like an awful lot of extra work than the way I was apprenticed/taught. The way I was taught was to minimize the amount of compound applied. VERY, very thin coats. The thinner, the better. In practice this works because the thinner layers dry very quickly. They are usually so thin that any imperfections can be skimmed off with a clean knife before applying the next thin coat. Multiple thin coats means you can work the job faster without having to allow for a long dry-period too.
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