Yeah except that hydroponics still need light, Underdark plants don't, which make them much simpler to grow deep underground.
We have an endless emergency industry, raising large amount of lifestock to sacrifice to Yss, Yss take animal sacrifice, and he reward people who sacrifice to him, so farming way more crops than you need, and raising animals you sell to people wanting to sacrifice to Yss for a boon, is a valid industry, and there will always be a market for more animals to sacrifice.
It's not the industry that's most rewarding, but with good tools and fertile soil, you can get quite a bit above subsistence, seeing as even if it's at a low price, you have buyers for all your surplus crops.
If the Fungus forge accept mundane plants as feed stock for growing things, then selling plants to be used as raw materials in it, will also be an industry we can put surplus people in, as we can always use more plant creatures, if nothing else because they can be used as soldiers on other planes.
So we have some industries people can earn money in, that have no cap on how many workers it can support, Yss is insatiable when it comes to minor sacrifices, and he always reward sacrifice, so dedicating an industry to feeding him so he will give more boons, is a way to get everyone in the Imperium paying jobs, doing something that benefits the Imperium.