It's not like we have exact examples of it yeah, but with language you can read into things. This is specifically a phrase that uses the terminology of planting, and harvesting in agriculture in a non-agricultural context. Typically that doesn't just pop up on its own in language. Implied meanings like that develop over time from more direct usages. In this sense its an ironic usage. Harvesting what you plant is a definitively positive thing in ancient life. It brings wealth, and abundance to you and your community. In this case its used as a negative though.

We can infer then that since the positive usage is the more obvious way to use it, that it came first. Why? Because otherwise it would confuse people.

Take a more modern equivalent. 'You made the money, now cash the check'. Cashing a check is good. Your getting money. If i tried to say it in a negative way people wouldn't understand as easily. Instead though if it started out as a well known positive phrase, and became something used ironically, which then spread as well. Becoming the dominant usage. People would understand implicitly that we were using it ironically.

Its important to note just how young the bible is too. You mention Akkadians and sure theyre older, but they are far from the oldest example of agriculture. In fact while the Akkadians lived over 4000 years ago, 2000 years older than the bible itself, organized agriculture has been around even longer than that still, 12,000 years. I would also argue this phrase might be older than that. Hunter-gatherers did plant things. They just spread seeds from plants they found, left them there, and came back around the next year during their migrations. They did this for hundreds of thousands of years. Its also a lot easier to forget to harvest what you plant if its something you did last year and then left it there. So the roots of the saying could possibly go as far back as that.

People back then did understand the concept behind growing things. They didn't settle into homes and do it full time largely because they were happy with their nomadic lives. Humans only began to farm the way we do now when there was a change in the climate and hunter-gathering became much more difficult. We also have evidence that those farmers diets were worse, and less diverse than their hunter gatherer ancestors. So yes while speculative the phrase could be that old.

When then would it have become ironic if it started out as positive? I think we have a pretty obvious contender for this. When people started living in cities. The people there likely would have known of the original phrase, but not really had any experience farming. They'd buy their food like we do, and not ever farm it themselves. To then take this phrase and start using it in an ironic way makes sense to happen around that time.

Now yeah thats a lot of speculation. The more specific you get the less likely it is your right. The basic thing though, that a phrase that would be understood as purely positive was probably originally used in a positive way, can be said with a pretty high degree of confidence though. Since for it to simply appear out of nowhere in an ironic sense would go against what we know of how language develops.

My favorite example of this is the term "Rizz" which has become common recently. Many people who use it dont even realize what its slang for. They just think its something that exists on its own. When in actuality its slang for Charisma. Cha"Rizz"ma. Now we cant actually prove when this happened, or how it started. But its pretty obvious that this is the case.

https://en.wikipedia.org/wiki/Rizz

"Rizz is a colloquial noun, used when describing possession of charisma. As a verb, rizz can be used to describe using charisma to attract someone, as to "rizz up" a person.[4][2] It is believed that the middle syllable of the word charisma was shortened to create the word rizz. Oxford University Press described this formation pattern for a word as "unusual". However, this formation pattern can be seen in the shortening of words such as refrigerator (fridge), and influenza (flu)."

It's a great example of how we can linguistically break things down to understand where they came from without direct evidence.