Odd.

Using Openwrt which pretty much all home routers are built on, all I have to do is tell my router which offset to give my subnets from the prefix and it does the rest.

Both for subdividing up the prefix from the ISP and my ULA prefix I use for internal devices.

I have changed ISPs I think 3 times with no ill effects. Plus it works when my ISP occasionally gives me a new prefix.

The only tweaking I had to do was when I went from an ISP that game me a /48 to one that only gave me a /56. I had been greedy and was handing a /56 to my internal router. I changed that to a /60 and updates it's expectations about which subnets it could hand out and all was good.

But I expect two layers of home routers without NAT is a bit of an exception.