For one, don't buy MVs, especially >>198940
. It's too expensive and the exchange rate from shells to pearls is abysmal, and who knows if they will release more songs like Edel Lille.
You can sparingly use boost tickets, or you can save them (although if you want to chase ranking, you might as well buy D4PASS). Same goes to coins, you can buy EXP tickets, but save enough to LB units.
Shells are mostly used to buy LB crystals, and they're obtainable from gacha dupes, D4PASS, and mileage. Silver tier gives more value than bronze, but gold is not worth reaching.