Fiberglass takes a long time to settle compared to cellulose. Cellulose
can get into area that fiberglass can't. Just make sure you have proper
venting. There should not be a dust problem as long as the cellulose is
not disturbed and there are no ducts in the attic. One issue is an
attic fan or bathroom exhaust fans. They can disturb the insulation and
create dust. One way I solved this was to take a large duct tube and
put it over the exhaust fan in the attic (this also solves the problem
of keeping the insulation 3 inches from the fixture). You can cap the
tube to prevent additional energy loss and run the exhaust hose through
the cap or side.

The cellulose will settle eventually and I think you should go up
there in 5 years and add a couple inches. As far as the entrance to the
attic goes, I would section it off and lay down batting so when you go
up there, a ton of cellulose doesn't fall out and get everywhere. Just
make sure you have proper venting between the rafters before you spray
that stuff in!