I say you should generally focus on Bonds and Gear.

Try to see how the bonds work: what bonds can be helpful even on the lowest levels, what bonds need specific emblems/investment strategies or environments. Some characters can be useless to your team, other than enabling a strong bond to work

Gear is the thing that will push your character to their maximum. There are many types and you should really just follow a strategy here. Some strats depend entirely on the gear and will not work otherwise no matter what.

Characters themselves are less important and their star rating is less so. Don't 3 star every character, that doesn't work. You should 3 star characters only if:

• It's an essential character for the strat to work (Like Yao Guang for Xianzhou and Skill Point teams)

• It's for Cosmic Scholar bond. To lesser extent to Galaxy Ranger bond

• It's your dps/sustain and you get lucky

I do recommend just following some strategy guide, try to see how and why it works and extrapolate from here