A very rational analysis of the costs and benefits of using greenhouses.

I note that your discussion does not entertain the idea of using compost to heat greenhouses, which you should. Compost was a critical means of heating peasants had available even when most impoverished, and there are theories that sleeping in compost heaps in Europe is what condemned Native Americans to plagues upon invasion by Europeans. Bacterial activity can raise the internal temperature of a well managed compost heap to 130 F, which is substantial heat available in a winter greenhouse.

Since you are in a rural area, there may be sources of manure that you could acquire in the fall, and compost to produce both heat and organic soil through winter, when the soil could be used to amend your dirt in the spring.

Also, you may not have considered using PVC pipes as the interface with the soil, and attaching other materials to the PVC above ground level, which may enable benefiting from PVC's durability in soil contact with other materials ability to sustain loads higher in the structure. Also, well compacted gravel is just as strong in compression as concrete, and drains much better.

Something I noted was that you did consider using LED's to provide light to crops, and simply building warehouses instead of far more expensive light transmitting materials. One thing that can improve the cost effectiveness would be to add large tanks in which various aquatic species are raised for food. Tilapia, prawns, crawdads, and many more species have been used in this way, and the waste from the aquatic species are fed to crops under LED's, or using sunlight by bubbling the aquarium water up to the plants, which clean it of it's waste products so it can be returned to the aquaria. There are good resources online for aquaponics systems, such as

https://www.nal.usda.gov/farms-and-agricultural-production-systems/aquaculture-and-aquaponics

and many more just a search away.

I am considering putting some greenhouses to work one of my clients has available due to their failing health preventing them from being able to use them after they invested in them. Your discussion really enabled me to properly evaluate their value as installed infrastructure, while the marketing of produce wasn't involved, that is more of a local potential and would necessarily be specific to my region.

Thanks!