A strange review, and I wonder if we watched the same film. A few observations.
It’s the 1770s, London [the film doesn't begin in London, but OK] is bustling with arguments about the slave trade and the status of young women is strictly circumscribed—in short, it’s not the best time to be Dido (Gugu Mbatha-Raw), the mixed-race daughter of a Royal Navy admiral and a West Indies native. As it happens, Dido grows up amid English privilege, surrounded by servants but hidden from scandalous view, her blond cousin, Bette (Sarah Gadon), getting all the attention [simply factually incorrect: a key narrative motor of the film is that Bette feels unwanted by males]. Belle, to its discredit, often forgets to supply the racism [examples? Because I can think of plenty of examples where racism is broached, both explicit and implicit: Belle's exclusion from dining, Lady Ashford and her son's open hostility], a serious mistake; the slurs are barely whispered, and you can’t help but think the character’s real-life story must have been a lot tougher [So there is only one form of racism? It must be either virulent verbal abuse - which is present, but you seem to have missed - or extreme physical violence? OK].
Closer to a special episode of Diff’rent Strokes [Inappropriately glib; also, in what way? In tone (which would be insane)? An allusion to the racial make-up of the ensemble? Because Belle’s milieu is hardly mixed. There is Belle, and then one black female help] than to 12 Years a Slave [Why this comparison here? Slavery as a vague theme is not enough to support it in this instance], the movie seems to exist to give its white characters belated moments of conscience [“its white characters?” Why are you tarring them all with the same brush? Tom Felton’s character is the embodiment of virulent racism, which hasn’t abated by the time the credits roll. John Davinier, the trainee lawyer, is absolutely driven by conscience from the first minute. The central narrative hook, meanwhile, is the beginning of the abolition of slavery. Of course working on the conscience of the highest judge in the land is a priority. So why do you present this as a pejorative?] Surrogate parent Lord Mansfield (Tom Wilkinson) comes to love the child as his own—and might even [why the snark?] have something to say about that from the bench when he rules on the landmark Zong slave-ship massacre [if you hadn't wasted your word count being glib, you might've been able to tell us a bit more about this key incident]. A smitten suitor [actually a fierce anti-abolitionist trainee lawyer] grows to respect Dido’s brain [this is patronising to both him and her - and the respect is mutual. Is your beef with the conventions of period drama now? Isn't the mixing of heavy theme and Austen-esque romantic style precisely what the film is going for?] The pretty deb cousin lashes out and then weeps in shame. [You're just not taking this seriously, are you?] You’d think being black was harder on them than on the title character. [Grammatically, this doesn't work. "You'd think being black" refers to who? Do you mean "Her blackness (she’s mixed-race, but OK) seems to pose more problems to her family than to her?" But that's not even the real problem with this, again, glib, sentence. Throughout the film, there are countless examples of the way that Belle's social conditions affect her - there’s a scene where she tries to tear her own skin off while looking in the mirror. But you simply don't seem to want to engage with any of them]