The rest all option is, afaik, never an advantage. The best you can hope for is no disadvantage i.e. your total number of players is a multiple of 3. eg if you have 17 players it'll cost 6 packs (disadvantage) but if you have 18 players it'll cost 6 packs (no advantage or disadvantage).

I think there's an error in the pack calculation in the OP because with my 21 players it would cost me 7 packs for a rest all. The rest all 'pack cost' also (I think) includes injured players in the calculation so thats a potential disadvantage.

But as the point of the OP is the reset time trick then that works, fwiw.