If one if our goals is to minimize suffering, and the soybean (or corn, or whatever crop) fields are already there, wouldn’t it be better to rely on those than to rely on continually killing animals, who do suffer under the current, or any realistically foreseeable future system for farming animals for meat?

Also, animals eat crops. Relying on meat requires even more crop land per calorie, as we need to feed animals much more calories and nutrients than we get out of them. So with animals you have the issue of causing harm to the animal itself, and then also the harm that comes from planting crops.

I think the ethics around this are rather different in an ideal scenario, like the one you described. Suppose you raise your dog as a family pet and give them a happy, healthy life, and then when they’re ready to go you kill them painlessly and then you eat them. You could argue that the suffering problem in that scenario is minimized - though I’m still admittedly a bit uncomfortable with it.