@andicniko
EDIT: After a factory reset and trying again, it seems it will work if 1) I state the DHCPv6 range in full (including the prefix), and 2) I state the subnet in the router advertisements settings.
For anyone else struggling to make this work, the specific settings are:
Services / DHCPv6 Server & RA / LAN / DHCPv6 Server
Range = [your desired IPv6 range in full, e.g. 1000:1000:1000:1000::2000 to 1000:1000:1000:1000::3000]
Note: DO NOT omit the prefix when stating the range. This is one of the issues that seemed to prevent my DHCPv6 server working properly (if the LAN interface is set to IPv6 Configuration Type = Static IPv6). By default, the range is stated excluding the prefix, e.g. ::2000 to ::3000. I'm not sure why this should matter, if the subnet field is already populated and aware of 1000:1000:1000:1000::, and omitting the prefix does no harm when the LAN interface is set to IPv6 Configuration Type = Track interface.
Also note: I also had some trouble keeping the "Provide DNS servers to DHCPv6 clients" checkbox ticked. It is ticked by default, but seemed to untick by itself when changing and saving settings on this page. When ticking it again and saving, it would just disappear. However, it was ticked after navigating to another page and coming back. So I didn't have an issue in the end.
Services / DHCPv6 Server & RA / LAN / Router Advertisements
Subnets = [your IPv6 prefix 1000:1000:1000:1000::/64]
Note: DO NOT leave this blank. This is one of the issues that seemed to prevent my DHCPv6 server working properly (if the LAN interface is set to IPv6 Configuration Type = Static IPv6). By default this is blank, and it does no harm leaving it blank when the LAN interface is set to IPv6 Configuration Type = Track interface. I'm not sure why this should matter.
I don't know if the above are supposed to be necessary or not - apologies if I'm posting something that should be obvious. But I hope that helps someone!