Right now, with a good slave overseer, slaves are considerably more productive than experts. As an example, I had a large farm on sweet potatoes; free colonist base yield of 11, expert farmer base yield of 16, slave base yield of 13 with a slave overseer bonus of 60%. I was Swedish with agricultural (+10%), had 42% liberty bells (+21%) and slightly reduced city health (+8%). This left me with col: 15.29, expert farmer 22.24, slave 27.9. Looking at the maths here, I assume the slave overseer bonus operates first, but is then rounded before the other bonuses apply, as that gives me close to the displayed output.
This shows that the slave is CONSIDERABLY better than an expert. I think there are two issues that could help keep slaves good, but not overwhelmingly so.
The first is that the bonuses compound; because the slave overseer bonus is calculated first, the effective bonus is actually much better than it appears. If instead the slave overseer bonus were only applied along with the other stated bonuses, then the bonus wouldn't be 60% multiplied by whatever other bonuses you have (subject to rounding), but instead added. Still a very good bonus!
The second is the potential of the liberty bell bonus. Here is where I feel there really should be a difference - liberty bells are where your free farmers really should be doing better! Slaves shouldn't get the liberty bell bonus - they aren't free! They aren't experiencing liberty.
If both of these suggestions were included, then at very high liberty bells, experts would be relevant again. Slaves would still be dominant in mid-game.
Now, I recognise there is of course another balance here in that slave overseers and slave hunters are needed to maintain a usable slave population
This shows that the slave is CONSIDERABLY better than an expert. I think there are two issues that could help keep slaves good, but not overwhelmingly so.
The first is that the bonuses compound; because the slave overseer bonus is calculated first, the effective bonus is actually much better than it appears. If instead the slave overseer bonus were only applied along with the other stated bonuses, then the bonus wouldn't be 60% multiplied by whatever other bonuses you have (subject to rounding), but instead added. Still a very good bonus!
The second is the potential of the liberty bell bonus. Here is where I feel there really should be a difference - liberty bells are where your free farmers really should be doing better! Slaves shouldn't get the liberty bell bonus - they aren't free! They aren't experiencing liberty.
If both of these suggestions were included, then at very high liberty bells, experts would be relevant again. Slaves would still be dominant in mid-game.
Now, I recognise there is of course another balance here in that slave overseers and slave hunters are needed to maintain a usable slave population