Isn't a prosperous Brazil good for the Portuguese union? One can argue whether slavery is good or bad for an economy, overall, but the simple fact is that the Brazilian economy was driven by slavery. Brazil has no interest in eliminating slavery. Portugal sees benefit in Brazil's triangle with Angola, and/or will need to accommodate both parts of the empire. Of course, if Brazil is capital, Portugal isn't the one calling the shots. Portugal will want to be the industrial part of the empire, which means keeping Brazil agricultural, which means slavery is a good thing.
Much, of course, depends on the nature of the union, how Brazil managed to remain capital of the empire, and how Brazil fared/grew in the 20s. Did Portugal lose a civil war/power struggle, as opposed to OTL? Did the military remain loyal to Joao/Pedro? Did Joao handle the post Iberian War period better, thus forestalling the liberal revolution? Has Brazil avoided the cataclysm of OTL 20's/30's? One has to presume Brazil, being the center of the empire, is the one calling the shots. They are going to want to keep slavery. A Brazil remaining capital is a Brazil likely more stable, with more sustained growth.
Uruguay is likely retained. Does this butterfly the Paraguayan War? This was the turning point, along with the USA civil war, for a rising call for abolition.
I think you probably see abolition more or less on the same time table. There won't be some massive early rush to abolition. The link with Angola will remain intact, which is conducive to slave trade. However, there is a world movement towards abolition. Brazil isn't immune to it, and will succumb sooner or later. I'm guessing later, which means OTL time frame.