That method works. You need a large board underneath to press the sag flush with the rest of the ceiling. You have to pick out and hoover up the rubble and dirt before pressing or some of it will get between the plaster and the joists or laths causing breaks. Once you have propped the board you can add metal lathing, screwed to the joists, to supplement the laths. Hammering should be avoided as the vibration may cause more cracking or the ceiling to fall.
Use a smooth board wrapped in clingfilm to prevent the runny plaster sticking to it. Or, I used laminated board which is glossy enough.
Pouring the plaster from above will also fill the cracks, holes and gaps, cast against the smooth level board.
Use a builders canister vac, not a domestic vac.
The longer you wait, the more likely it is to fall down, which will be terribly filthy and far more work
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