I invested in some bronze characters, which still have utility for doing bronze prize fights. Some bronze characters are really good and some have even better signature abilities than some golds. Most notably, Bad Hair Day Filia is my favorite bronze. She gets a chance to bleed on attack, which is incredibly useful against invincible and unflinching enemies. The back-dash after 4 taps is almost a guaranteed evade, so you are relatively safe on block.
They still help you in story mode, because some quests are "Bronze only", so don't be afraid to pump a little bit in bronze if you don't have any silver or gold.
I only level up blockbusters and special moves to 3 so far for the extra ability. You should save your gold for character upgrades- gold characters cost a lot to upgrade!
For signature abilities, I max the ones I like. For example, I have a Parasoul Primed, and her ability to gain enrage on tear spawn is absolutely worth maxing. Other abilities, like Diva Intervention's...not worth getting.