IPv6 tries to solve this with path MTU discovery.

Yes, but IPv6 is still at a higher level than Ethernet, Wifi, et al and is therefore subject to the limitations of the lower level framing

Sure, I mean that's what pMTUd is all about. One big difference with IPv6: Routers can't fragment packets. They either send or they don't.

I thought so too, but apparently there is an IPv6 fragmentation extension and it's implemented by several operating systems.

Only the endpoints can fragment.

Sure?

At this point 1500 is the standard, we can’t ever hope to increase it without a way to negotiate the acceptable value across the entire transmission path - that’s what IPv6 gives us.

I'm not sure that negotiating the acceptable value across the entire transmission path is a reasonable thing to do. I'm not sure that IPv6 _should_ be aware of a minimum/maximum MTU of underlying transmission path particularly since that path can often change transparently and each segment is subject to different requirements.