It's the inelegant management of slaves that's the issue, and the fact that this headache contrasts with 2.2's mandate to streamline and automate a lot of the nitty gritty of pop management. The fact that it disincents separating planets on a slave/free basis precisely in an update that highlights planet specialization is weird.
A couple ways I can see to improve the situation:
1) Just auto-enslave by job and slavery type, as caste system used to. If a species has chattel slavery, any pop that works a farmer/miner/technician job becomes enslaved, and loses the condition if they move out of the job. Domestic servants are also enslaved if they work as entertainers, or if they become unemployed etc.
A planet with lots of slaveable jobs will have lots of slaves. One which doesn't, won't. If auto-slavery proves to be OP, just adjust the production bonus from Slaver Guilds, or make the ratio of enslaved/free workers less than 1:1, or insert some other drawback.
2) Insert a planetary decision to increase the slave pop spawn ratio or decrease it. You can selectively create more slaves on worker-job worlds, less on specialist job worlds. Insert Energy/Influence/Food cost to balance.
I think the reason why they didn't do 1 is because of the job auto-fill system; either you'd have angry freed slaves every time a job opens or you'll sit there with the job unoccupied until a new pop grows.
2 is probably the better option. One idea I had was to mark clerks (which are the last type of job slaves can fill) as a special type of job for Slaver Guilds, one which eventually leads to freedom as they make enough of their own money to buy their freedom/pay off their debts.
In the meantime the best option that I've found atm is, once you have another species or two enslaves, is domestic servitude. Unemployed slaves will become servants and produced amenities, and since slaves take up less housing, you should still be able to have enough free pops to fill normal jobs. With a couple species that can be chattel slaves, you should get away with that w/o losing too much production from the lower slave output domestic has.