Forms, HTTP implementations, public API surfaces, and all for what exactly. Introducing a new verb for this feels profoundly misplaced

Idempotency is an important attribute for correctness. Yep, you can document that POSTing to $ENDPOINT is idempotent, but you can't communicate that to caching layers throughout the network. QUERY, by definition, is idempotent and cacheable.

Great point. I wish more people realized that intuitively.

Does anyone know if this approach works at significantly larger scales? Curious about where it breaks down.

Larger scales like what? I expect that everywhere you currently cache GETs you can cache QUERYs. But does caching GETs work at scale?

At least support - or lack thereof - for a new verb is unambiguous (compared to changing the semantics of GET)