My Chang tried to warn you, Sanderson turned Modern Audiences on this one, with transgender and non-binary minor characters, and two POV characters that are now not just out gay, but gay for each other, despite zero indication of that prior, in a medieval themed universe where Inheritance of Titles is an explicit plot point.
Personally I didn't find the beginning unreasonably slow, but then again Rhythm of War is a low bar to clear.
And yes, the final 10% entirely flips the table, even if it the most explicit "To Be Continued After This Other Project I'm Doing" I can recall. Actually, Projects - in addition to Mistborn era 3, he's also going back to Elantris and turning that into a trilogy.