> Yes, it does. You just have each of your routers (wired and 5G) advertise the /64 prefix delegated by each of your ISPs. Your hosts will self-assign a v6 address from each prefix.

> To control which link the traffic uses, you just assign router priority in the router advertisement (these are all standard settings in radvd.conf).

Have you done this? Did it actually work for you?

When I tried it, clients would regularly send to router B with an address from router A, and often ignore the priorities. As I understand the RFCs/client behavior, the router priority field is only relevant if multiple prefixes are in a single advertisement, otherwise most recent advertisement wins.

Once you need to aggregate the advertisements, you may as well NAT66, cause it will be easier.