In my opinion, the collected resource should be put in maybe like a storage or granary building once the slave gathers a certain amount of the resource (in the meantime while the slave gathers the sufficient amount of resources to take to the storage place, they can store it at the farm itself, until it reaches that amount), once in the storage place, the resource should be available to the player for use.
The storage place should have a storage limit and other traders and merchants should be able to get the resources from the storage place and turn it into usable goods. For example the baker will buy (bought from the player of course) wheat from the storage place and turn it into bread, that will be sold to everyone else. The farmer for example should receive payment from the player when they put their Harvest in the storage place.
Once the storage place is full, the player should have the option of purging some resources for more space or selling them to neighboring towns. Note only raw resources should be stored at storage places. Processed resources should be stored at the respective shops (which should have a storage limit of course) and processing buildings. The player should be able to trade processed goods with other towns too imo.
The player should be able imo to get coin by sending out tax collecters automatically (after some game time that would represent a month or whatever time span), the tax collectors should collect a portion of the people's money (the amount collected is dependant on the whether it is high taxes, medium , low or no taxes of course).
Players should be able to use coin and weapons to train soldiers and to buy needed resources at a high price if there is a shortage of something.
Finally, I think traders and merchants should be able to strike if their happiness is to low and slaves should be able to have an armed uprising if their happiness is too low.
Owners of slaves, like farmers should have the option to overwork slaves to get resources faster but this should steadily decrease their morale, the slaves should be adjustable, with lower workloads increasing their happiness and higher workloads increasing their unhappiness and chance to cause an armed uprising. Slaves that cause an uprising should attack free citizens, soldiers and loot and damage buildings.
Maybe once a certain amount of slaves are killed then they should be forced to go back to work.