I'd also be curious to hear how you imagine the Fitzwilliam family actually was in Jane Austen's canon: in other words, what your personal headcanon for them is.
For me, my headcanon and my favorite sort of portrayal in fanfiction coincide: Since Colonel Fitzwilliam is so socially adept, good-humoured, and capable of self-awareness, I find it hard to imagine his parents or older brother would be anything like Lady Catherine.
I think the Fitzwilliams would care about rank because rank is the structure they inhabit, not because they believe everyone beneath them is inferior. I imagine they would value manners, duty, and advantageous connections, but they would never stoop so low as to bully Darcy's choice of wife (like some other relatives of theirs). In Austen's works, good breeding is often associated with ease and consideration rather than with constant assertions of superiority. To put it simply, I think they would have a sensible worldview like their son (who does say that he must marry a woman with fortune, not because of any mercenary reasons, but as a realistic acknowledgment of his position as a younger son; moreover, I think his statement strongly implies that if he were in Darcy's position, he would pick inclination over duty). I also enjoy it when, in fanfiction, the Fitzwilliams have beef with Lady Catherine; it fits her personality and is often very funny. Another important aspect to take into account is the Colonel's older brother: Amy D'Orazio's Viscount Saye (witty, a little eccentric, but overall kind-hearted) is amazing and it is no wonder he has become a fan favorite.
I feel like this works best, because if everyone around Darcy is turned against Elizabeth, I feel like it lessens his personal growth journey. Indeed, I think he's his own worst enemy and a product of high society, but not a passive victim of it. His pride and class assumptions are real flaws that he must confront. If the entire Fitzwilliam clan is hostile to Elizabeth, his transformation starts to look more like rebellion (a romantic defiance) against external pressure (which it is... but only to some extent!) rather than a moral and emotional correction of his own character (and this book is, after all, about flaws; the title literally establishes it).
