The biggest, at least in the US, is land use and allowing housing to be built close to jobs, in multi-use zoning, that allows people to go about their day without a car. That's the political third rail that even environmentalists fear to touch. Suburbia is unsustainable and we must allow more urbanization, and that will make all the rest of the challenges far easier than even stopping population growth. Because urbanized populations consume far less resources, need far less carbon even with current technologies. And it will make the transition to renewables much easier.
Meat is an order of magnitude less impact than our bad land use, at least in the US.
And nuclear is a lost cause. Even if we double our construction rate every year, it's not going to catch the tail of storage, solar, and wind in the time span we need to shift off carbon. And our current nuclear fleet is aged and ready to be decommissioned, even replacing our current nuclear fleet would require huge growth in an industry that doesn't know how to complete projects, much less scale and get cheaper. This is not a political problem, it's an industry problem.
Population growth, similarly, isn't much related to our immediate climate action. We need static change in the next two decades, and telling everybody that there are no more kids entering the workforce in order to actually execute this transition would be disastrous. Say no more children are born for the next 20 years, and population drops, I don't know, 20%. I don't see that she distribution being set up to tackle the future challenges of the world. Population growth is already peaking, and the best thing to accelerate that is to industrialize the high population growth countries with green industries, and urbanize their populations.