Well, river civilizations have their own set of problems. As
@veekie pointed out, early on fruits and nuts are more nutritious than rice or grain. It took thousands of years for human beings to genetically engineer plants and animals for their own use, so while the river civilizations are growing faster than us, they aren't growing
exponentially faster than us. In both Brushcrest and Makar's case I think that we're overlooking how vital fishing and fishers must be to their food production.
Also, rivers flood. Efficient and sustainable agriculture means irrigation means sustained efforts to tame the waters. The degree to which a neolithic civilization can do this is questionable at best. And expanding too far south of our effective power projection means exposed targets for raiders like the Makar. I think we can do entirely reasonably for ourselves by expanding into the mouth of the valley but no further, while also finding some way to defang Brushcrest and the Makar so we can assert local hegemony.
But all that depends, ofc, on how things transpire in the update.