Provisioning an IPv6 prefix with redundant WAN links - best practice?

Hey all, I have a puzzle I need to wrap my head around and need some clue. I'm way behind the curve on IPv6 and need to get with it, big time.

The quick scenario is the end-user/customer has redundant connectivity to their office with the same ISP, but the two connections are different speeds and tech (fiber, and xDSL bundle). They want to have a /48 allocated to them and have it reachable via both connections. I'm a little stuck on what the "best" way to do this is and need some pointers. It's for a business customer and the PE and CE boxes we're dealing with are cisco gear.

To my knowledge, there's no need for an SD-WAN type of solution. They'd like to be able to load balance over the two, but I'm not sure how to do that in the IPv6 world "correctly."

So, in the simplest of scenarios, with fiber being the sole primary path and xDSL being the backup if fiber dies, what's the best way to set this up? I'm reading up on IPv6 Prefix Delegation and DHCPv6, and basic static routes over local point to point subnets (similar to IPv4 /30 subnets from PE to CE on each link). BGP could work, too.

In your guys' experience, what has been the "best" way to do this? Or maybe a better question is, what methods were the worst in hind sight and that I should avoid at all costs? Any clues would be hugely appreciated.

Thanks!