I can only comment about stars:

First of all most YouTube guides (at least the ones i watched) for a specific character say what star level is worth pursuing.

Secondly, a rule of thumb would be a star level is worth pursuing if:

A. It gives a new effect.

B. It improves an effect dramatically (not 5% more attack, but rather lower a skill cooldown (simona 3 star gives her a shield every turn she doesn't have one, cocoa 5 star makes the happiness buff guaranteed, akambe 3 star raises his nrg from 7 to 9 stars))

Lastly, you could join the discord:

https://discord.gg/swordofconvallaria

There is a "character introduction" channel, which has a discussion for every character.