Humankind has a weird mechanism: you can ransack yourself. Though the original reward of ransacking is pretty neglectable, it can be extremely rewarding with minimum preparations that can be done even in roughly turn 40. The mechanism obviously does not make sense, how can you keep robbing yourself and get richer and richer without limits? Rob, in my understanding, never creates new stuff; it just changes the ownership of the items being robbed. However, robbing in Humankind does create both money and population from void. In fact, that's not the problem of humankind. Civ6 players will remember how rewarding pillage is and know the best way to make money is to rob your poorest AI neighbor, despite he does not have even one tenth of the money you gained from pillaging.

Now let's talk about how to get rich from ransacking yourself. The necessary requirement is one of two things: either you have something that grants you bonus money from ransacking, or you have much larger influence generation. The bonus is the war slave civ that grants you +1 population from ransacking. For me, the easiest way to fulfill necessary requirement is waging war against one of your neighbors and actively ransacking everything he built outside his walls, like outposts, strategic and luxury extractors. If do not feel like wars, you can also use some scouts to ransacking extractors of (non-violent, if you do not want to handle the barbarian chariots) independent people. A few ransacks in the early game will easily award you a (golden) thief badge. Then you can park a scout at one of your horse of copper deposit, ransack it to gain juicy money and +1 population, while immediately buyout the next turn so that you can ransack every turn. The costs of buyout is significantly lower than the costs of ransacking, allow you to create free money and population simply by repeatedly robbing yourself. And since you are ransacking yourself every turn, you are very likely to renew your golden thief badge. Congratulations on being the richest thief in the world that has a special love of robbing yourself!

Another way to do this is to choose some culture that provides you more influence than you can spare, then you can park your scouts at one of your outposts, rob a horse tile every turn and influence-purchase it immediately. This will grants you lots of money at the cost of a little influence. I once did that with Ming, since I simply do not know how to spend the +2k influence Ming grants me with its powerful emblematic districts.

This mechanism does not require you to choose any specific culture. But some cultures may be helpful: Egypt to reduce costs of extractor, Carthage to reduce costs of buyout, Mongolia or Norsemen to increase yields of ransacking. But all these cultures just affect how much more you gain from robbing yourself. Even you choose non of these cultures, robbing yourself is still profitable in terms of money and population.

I have attached a photo showing this. I'm at turn 43 at classical era with golden thief badge, I gained 164 money and +1 population from ransacking my horse tile while buyout it only costs you 92 gold. The costs of buyout and rewards of ransacking scales simultaneously, which means you will benefit from it the whole game.

One extra note: ransacking will destroy all trade routes to, from and passing through that city. Make sure you do not destroy your own routes. As for AI's routes, you don't care about it anyway right? If you interrupts AI's purchase of your resources, Ai will just buy it again which grants you extra money. As for diplomacy..., well, you do not like those stupid guys who attacks you immediately when they see your right?

If devs are to solve the problem, simply dramatically reduce the rewards of ransacking yourself should do the job.