One /64 prefix cannot be split into two /63; actually /64 should not be split further.
For the choice of suitable ip6assign values, the prefix size received from upstream should be considered, rather than the previous ip6assign value of the lan interface.
Am I misunderstanding something?
I agreee it is important to check the values of ip6assign, even more so if routers are cascaded (which was not discussed there).
I also think the title "Routed AP" can be improved. I thought it was some AP mode (ie without wan interface), but it seems to be a router with a separate WiFi network.