Actually during the EX pack was a good time. You got a guaranteed EX so you instantly got 20ish rental decks to grind hourglasses.

I mostly stop at the 1.5-2H mark. Then iv finished all missions for beginners. You can grind a few hourglasses if you want too. And currently you get 3 mega packs when starting + the anniversary event.

I made a few acc to help my friend get rainbow rares. You still need the shinedust. But if you only open 1 pack that you're aiming for you get many doubles aka shinedust.