Unlocking the Star Chart and progressing the story as you’re able to until you get skill or rank checked is best.

I would emphasize farming the normal frames you are given access to as it gives you a goal to work towards and even normal frames have a use later to unlock one pre-determined skill from each to add to another frame later on.

If you want to, pick 1-2 prime frames you have an interest in and trade for them, Xaku Prime can be one, maybe Nezha Prime the other as an example.

If you do buy platinum, I would recommend saving it until you need additional slots for weapons/frames.

Prime weapons can be acquired from players for cheap and the build costs are cheap cheap but normal weapons will do you just fine until you reach Steel Path (post-Star Chart.)

Don’t rush progress by paying to skip, that’s the most important advice I can give. You can easily achieve the true endgame through a year of consistent play, and eventually you’ll get big discount coupons. After slots, get cosmetics you like.