You raise a good point and ending slavery should be a top concern.
I'm in Portugal, and we've had a few cases of slavery and abused foreign workers here as well, which is shameful for us as a nation, but we have many good examples of good practices where applying technology improved production, lowered waste, turned out better product for the consumer and allowed for less use of hand labour but with higher salaries.
The starting investment is high but the subsidies you mention could/should be converted into low or zero interest, long term loans and the money recirculated towards more improvements in the sector.
Greenhouses do consume immense ammounts of fertilizers but water is better manageable under those conditions than sowing corn, which is well known for being a syphon for water and agro chemicals, and usually leaves the soils destroyed after a few years of intense farming.
Any change for a better model done will a step forward. Cattle, as it is raised today, I don't find it sustainable.