If it's MAC based SLAAC, it'll be stable, so you could use it. Otherwise you add a stable address. You could use DHCP if you want. If you're using a backup ISP with dynamic DNS, your failover should be the same as ipv4. You can already have ips from both prefixes on the interface. You're also way past clueless home user stage if you have that setup at all, so I can't imagine ipv6 is the stumbling block.

Things would be nicer if NAT66 were used by default for home users though so the question of prefixes would disappear, and it'd perhaps match the "more advanced" home user ipv4 mental model. You'd just use e.g. fd00::2 as your server address.