I understand that it must be stressfull and frustrating. You might have the feeling that you don't understand something. But I'm not here to put you down with knowledge, I'm willing to share almost everything.
In term of storytelling - and when I'm sure of something - there is close to no chance that I can be wrong. We are not talking here about theorizing.
When I say - for example - that a post is ALWAYS meaninfull in term of thematic, this is no just a theory, this is a deduction from real parameters in the story that points toward this direction and a logical and NECESSARY principle of storytelling.
If post were not meaningfull for the story, most of the stories of the strawhats would be bland.
Meaningfull doesn't mean that there must be something like an entire story behind a parameter like a post, but that there must be some storytelling behind the parameters.
- Luffy is not the captain simply because. He is the captain because he made a promise.
- Zoro is the first mate because his entire story is focused around the symbolism of honor and the confidence that there is between him and Luffy (same for his combattant position, but this time because of his story with Kuina)
- Nami's post is not just a Navigator, she is the embodiement of what a navigator should be, her thematic are all about the notion of science and knowledge. You can also add that her story and relationship to the sea is put to the extrem, she literally FEELS the climat.
- Usopp is not simply sniper out of nowhere, he is a sharp shooter because this is what is Dad is, Usopp wants to feel as close as his dad as possible and therefore become the same proud warrior of the sea
- Sanji's relationship to food is highly linked to is relationship with others, the way he was treated and the way he treats others. Sanji is not a good cook because he can cook well, he is a good cook for Luffy because he is good guy that will cook even for his ennemies if needed.
- Chopper post is higly related to his past and the thematic of him not trusting himself enough.
- Robin is very interesting, her post is archeologist, but this is what she does with it that is important. Her thematic is purpose, just like her role on the ship. Robin has a purpose and a dream, to find the truth, and to find that truth she must first find the way which is represented by the poneglyph and the ROAD poneglyphs.
- Franky's post is shipwright, but its basically taking care of the Sunny. Franky thematic is all about being proud of his creations, the Sunny is his magnum opus, he will therefore take Sunny to the end with pride.
- Like I said previously, Brook post is all about bringing the joy in life to others, that's what a musician do and that what all the story of Brook is about. The joy of being alive.
- Jinbe's post is related to his characterization. Jinbe is the guide of the crew, both literally and symbolically. He is the one that will bring back the moral and be behind, watching. He is the one that will guide the ship toward its destination
As you can see each post is related in some way either to the characterization, to the story, to the past or to the dreams of the protagonist. This means that EACH post is MEANINGFULL in term of storytelling.
So, if you look for a new strawhat, you must FIRST look for the meaning behind a post before saying "X will be shipguard or combattant"