Ah right, thanks Kotono.

Did a a few trial runs to test rerolls after my main account didn't get the 3* I wanted out of the 3 (Got the healer girl instead of Isabelle), and I think the best strategy is simply skip through the story as quickly as possible until you get to the gacha, and then use your discount 2 pulls in step up to try and get a 3*. Rinse and repeat. Takes only a few minutes and you have a guaranteed 600 diamonds for pulls from the matching minigame on the startup screen.

Ran into a concern on my first run through, and that was shards for 2*+ rarity character evolutions.