Slavery needs to be more punishing in the late game
I’ve been playing as the Ottomans, Brazil, and some middle eastern states recently, and the overwhelming feeling I get is that the benefits of abolishing slavery outright are almost exclusively the internal political benefits of 1) shifting a small amount of power away from the landlords (which usually have to be made weak in the first place to get rid of slavery), 2) allowing you to adopt multiculturalism, which is good but only marginally stronger than subjecthood for multi-ethnic states with a large population, and 3) removing the small hit to influence, which is functionally irrelevant for most of the game.
The main economic drawback of slavery, namely your slaves not having an income and thereby not paying taxes, can be made up for in the mid to late game with graduated taxation which lets you appropriate the extra dividends to landlords.
In exchange for this, it guts the productivity of your sugar, coffee, tea, banana, cotton, etc. plantations with no real way to make up for it by removing access to the “violent treatment” production method; automation just increases total output on top of slavery’s bonuses, rather than making it less valuable in the first place.
Slavery should be something that’s extremely punishing to keep in the long term, but right now the game makes it more punishing to remove it than it is to keep it even if you have the political power necessary to remove it.
Some options for the devs:
A production method that increases plantation output to a level comparable to slave plantations, maybe as a bonus for cooperative farming or commercial agriculture (to represent increased worker motivation and efficiencies of scale, respectively)
A mid-to-late game journal applied world-wide surrounding the abolition of slavery, giving production bonuses to nations that use standard labor when using automation or other enhanced production methods. This journal should create diplomatic strains between countries that abolished slavery and those that did not, in whole or in part, and ideally create associated events to amplify pressure towards abolition.
Increase the number of workers required per plantation for buildings with the “violent treatment” and “exploitative practices” methods, to represent the inefficient and extractive nature of the institution without completely removing its immediate-term economic value. Basically, the idea is to increase total revenue per building while decreasing per-pop output