I'm no dietitian, but for the "cheap, broadly available protien" I think beans and lentils fit the bill. I do not know if they are sustainable, which you mentio

I'm no dietitian, but for the "cheap, broadly available protien" I think beans and lentils fit the bill. I do not know if they are sustainable, which you mention as a requirement in your post, but surely plant based is more sustainable than meat.