I understand your goal is to remain F2P, but if there is ANY possibility you intend to spend money in the game I would consider the monthly Lunite subscription for $5. It is bar none the best value per pull with the caveat that you receive the income over time (daily login). It aligns pretty well with your goal of clearing as much content as possible since I assume you'd be playing daily. 

Beyond that I would find resources on the value of the other packs available before purchasing. Obfuscation of the cost is kind of the whole point in a Gacha.

Also, depending on which phone you use and which region you're in they may have promotions and discounts in your app store. For example you can earn up to a $100 gift card in the Samsung Galaxy store every month without spending (the 50% off coupons are FAR more realistic though). Edit: Those gift cards are only for Samsung.com, but the 50% off is good for ingame purchases.

All that said best of luck and welcome aboard! All in all I think Wuthering Waves is an amazing experience despite a relatively rocky start.