It's plausible that a larger brain developed because it offered advantages. But later that brain was optimized without increasing in size.

For example, my current desktop computer is the same size as my stone age IBM PC.

There’s a reason we call them anatomically modern and not just humans. There is a period ~35kya where we get “Modern man” and it correlates with increased brain activity.

Things like “complex” cave paintings, tally marks etc take off right after (in evolutionary terms) as we marched towards farming and the written word

I do believe that "anatomically modern" includes brains. https://en.wikipedia.org/wiki/Brain_size#Variation_and_evolu...

Can you provide any sources for claims about brain size? I admit I'm not an expert, but I've never heard claims that brain size significantly increased in that period.

A few other facts that I can't say rule out what you're saying, but make it counter-intuitive:

Mitochondrial Eve was 150,000 years ago, Y Chromosome Adam was > 200000 years ago. Aboriginal populations in Australia are descended from populations that reached the island close to 50000 years ago. The Bering Strait was crossed close to 20000 years ago.

P.S. I do agree that saying fire might've been discovered within the past 10,000 years is unbelievable.

Edit: I belatedly see you might be talking about other changes to brains that aren't associated with increased brain size. Still, I'd ask whether there's evidence that anything about our brains changed 35,000 years ago, as opposed to us having accumulated knowledge and culture over time.