alt Hacker NewsAha! Mansfield Park has been my favourite, since the 1980s. Most Austen fans seem to dislike it, and see the central character as weak and insipid. But, I see it as about code switching and timidity, she's clearly an introvert being bullied by the family.
The intrusion of Caribbean wealth is slavery. Edmond should be more overtly concerned about his families wealth, although to be fair it was mainly dissenters who did this, not the recipients of a family living.
She's on the money for naval preferment. Without help, commoner middies didn't make the crucial step up towards post captain. And for preferment, sexual favours by a sister would be common.
It may be important to note that all of Austen's novels are set up and kept going by an essential flaw in the hero characters. And Fanny Price is the ultimate anti-hero: indeed, she is hardly a heroine, she doesn't act, at all, she has no arc, she just clings to the first thing she encounters. While the world is moving and swirling around her, she doesn't move at all, not for lack of opportunity, but as a character trait (and, at times, by sheer luck, as in the theatre episode, thus earning her uncle's regard). And, in the end, it's all for nothing: while she got everything, she may have wanted to embrace, and even more, she is untouched by it and (quite literally) still where she started. It's quite literally about first attachments, not just in the domain of romance (like in Sense and Sensibility), but to about everything. Or, rather, first possessions, as in the first room, she may call her own, the first person, she talks to, etc.
How Austen constructs a plausible environment for such a character and what she does with this world and its characters is quite astounding – and hilarious. And, as you said, there are actually serious topics discussed.
Even more astounding is maybe how modern adaptations try to render this as "how our quick and cunning girl stirs up that lame family and wins everything."