Finally cleared Stage 3 ㅠㅠ

Finally cleared Stage 3 after a lot of grinding. With energy recovering only one per hour, it was a true test of patience... sob sob ㅜㅠ I feel like I'm finally getting the hang of it. Early on, it seems like you need to aim for 500 stamina while increasing income a bit to see the money grow. Then, after an upgrade, push stamina to 1k, slightly increase income again, and repeat for the next upgrade. The key seems to be getting to the next upgrade as quickly as possible while also increasing stamina... I'll know more after playing more, but definitely don't press the 'buy' button...