This appears to be a problem that is solving itself. Developed countries have birth rates below replacement and more countries are becoming developed (with the same trend happening as those countries develop).

That’s all well and good if you reach stability below a level that is sustainable. We aren’t going to be anywhere near that from an environmental point of view.

And whybdo you believe that? We will never surpass 12 billion by most estimates. Where is the sustainability point?

The sustainability point is where we can survive as a global population without destroying and depleting the resources of the planet. What point that is exactly is impossible to say because we have never done proper research (as far as I know), but what we can say for sure is that global population passed that point a long time ago. Mass extinction, overfished oceans, deforestation, saturation of the environment with chemical pollutants (not limited to carbon), soil degradation from nitrogen saturation (and the nitrogen runoff that has created massive dead zones in our oceans and other waterways) - the ways we have destroyed our planet in order to sustain our overpopulation go on and on. As evidenced by the downvotes to my original comment, the unfortunate reality is that many, if not most, refuse to accept reality out of religious-like delusion. Some worship at the altar of technology, some believe that its immoral to see a moderation of human population, some worship at actual religious altars and believe its blasphemy to even discuss the issue, and some blindly accept conventional wisdom that we need an ever-increasing population in order to sustain the economic ponzi scheme that modern society rests on (despite the fact that we have a massive supply of surplus labor).

One more time: we cannot destroy our planet, even if we wanted to. We can make it less comfortable to live in some places, though.

Also "we don't know where the point is, but we have passed it" is a very bad argument. "What can be asserted without evidence can also be dismissed without evidence".

Do you think those 12B people are going to pollute the ocean with less plastic or agricultural runoff?

The last time there was a "population crisis" fertilizer was invented the problem vanished. Currently GMOs have shown they produce massive improvements in yields and we're still in the early phases of this research. So it isn't exactly naive to think that this won't be an issue (though I'm not saying you should completely dismiss it).

As for land, well Europe is the same size as America and has twice the population. And neither have anywhere near the density that China or India does. So there's quite a bit of evidence that we shouldn't be overly concerned about these issues.

And what looks like the best way to solve them is by helping other countries develop more rapidly (which is actually a bonus for tackling climate change too!) but this is a pretty unpopular opinion.

How do we go about dealing with the massive overpopulation?

As per sibling comment to yours, society wide development seems to lead to lower birth rates. Lift the masses out of poverty.

One of the simplest ways is to offer economic incentives to people who have fewer (or no) children instead of the opposite (which is our current policy).

Soon enough we will have the opposite problem to worry about.