I'm having a tough time seeing how the gene-level selection works in your account, though. Genes don't inherently care about memes [...]

Memes have benefits to genes. Copying avoids the costs of individual learning. It allows you to obtain lots of good ideas quickly, by copying them from successful individuals. Good ideas are important. They can be mastery of fire, hunting techniques, love songs or negotiating strategies. Thus we have meme-spreading adaptations: our incessant babbling, our ultrasociality.

and larger cranial capacity comes with major disadvantages that require overwhelming genetic selective advantages to compensate.

These costs apply equally to any theory of human cranial expansion. Not much besides the enormous benefits afforded by cultural inheritance can reasonably expect to pay for them.

Certainly our big brains are the result of a sexually-selected arms race - but what was sexy was a GSOH, being able to sing love songs, etc. IOW, the products of cultural inheritance.