Here is one that gets a bit ugly. Lets say we chain a bunch of LEDE based routers together. Lets also enable a nice numbering system in IP4 and IP6. Lets enable DHCP-PD through the network, because our ISP was generous with a /56. The IP4 is only static and is easy to control, see table below. Its tedious, but easy. The problem is in parallel the IP6 addresses compound in the ULA region. Each router DHCP-PD the ULA above it, and gets its own ULA from Global settings in Network UCI. There seems no easy way to control this mass cascade of ULA, and get good fail over.
(1) If router above is unavailable, then next tier router uses ULA (network global UCI) to begin assigning addresses to clients and subnets.
(2) If router above is available, then next tier router expires its self-generated ULA and subnets, and gets ULA from delegation above to then re-delegate to subnets.
(*) Bonus: make some parts optional
IP4 WAN Name Serves Subnet
(NAT) Router-1 172.16.0.1/24
172.16.0.2 Router-2A 172.17.0.1/24
172.17.0.2 Router-3A 172.24.0.1/24
172.17.0.3 Router-3B 172.25.0.1/24
172.16.0.3 Router-2B 172.18.0.1/24