I will defer to your experience re the utility of tiled-but-still-GeoJSON as a sensible middle ground. I think you're right that we haven't seen this as an area that merits significant attention--it's sort of "not worth optimizing yet [geojson]" or "worth optimizing [MVT]". But I can see how there could be middle grounds in some scenarios.
PMtiles is what I had in mind when I mentioned ergonomics. Brandon's delivered a very compelling approach, as I hope I conveyed to him at CNG earlier this year. The lack of fully specified behavior re range requests is a lingering concern, as you acknowledge, and there are some other areas like incremental updates where having a huge mess of tiles still looks pretty good. But I think it's fair to say that it's overperformed as a solution, and I understand why people are excited about it and the other cloud-native geo formats that have emerged in recent years. Decades ago, Mapbox founders were at DevSeed writing PHP--there will always be some sympathy around here for "just upload it" as a deploy story!
I can't talk about the optimizations we are investigating, but I can at least acknowledge some of what makes the problem so hard (and the update schedule so slow): MVT is quite good, and backward compatibility is a pain, especially when you're optimizing for bundle/binary size (a particularly hard problem when your competitors get to preinstall their libraries on every mobile phone in the world) and working with a customer base that's preintegrated, in every way and to every extent imaginable, with an existing ecosystem. There is a reason people still use shapefiles! Though I hope MVT's reputation remains a bit better than that...