hey,
So I'd say there is equipment out there that works, as expected, but as seen in this thread, plenty of equipment that doesn't.
Latest OpenWrt releases include https://github.com/sbyx/odhcpd as DHCPv4/6 server. This enables hierarchical PD on these platforms, ie. subdelegate /64s from the /56 PD prefix you received from SP.
It's not the most configurable thing at this point but it does the job. -- tarko