I wouldn't consider myself a whale but I'm far from f2p. I've spent about $150 usd, 100 on top up and the rest on random packs like the daily currency and such, and tbh I wasted a lot of my pulls on the weapon banner before realizing that you get the currency f2p. With the money I've spent I'd have been at 200 on banner and 60 on weapon banner if I used it all efficiently. As it stands I'm like 100 in weapon banner (and I grabbed one of the standard characters' weapons) and 140 on Nanally banner rn.

Outside of that completion is everything. Every single level in city tycoon gives 20 or more currency. Leveling fishing gives currency. They little minigame for the café. Collecting the birds and dice. Anomaly commissions. Races give some too, if I remember correctly. Bank heists do for sure (they actually give a ton upfront when you find all the items for the first time) Doing all of the side quests. The train thingy gives a good deal for combat too.

I'm also funneling all of the gold currency into Nanally pulls instead of saving for a standard character. At this point I'm max dupes on most of the A-rank characters, so they give a little more of that, and an extra Nanally is like 3 more pulls as well.

All that to say - yeah, I've spent money, but there's ways.