So - does that mean that it's correct to say that '2a02:0390:6c03' is my 'static IPv6 address' (though of course it's only a prefix)?
It's your
static IPv6 /48
prefix, rather than
address, yes. It's up to you (and your router) how you make use of the address space under it. Most times, the DHCP6 on your router (and router advertisements from it) in conjunction with your LAN devices will result in them auto-configuring themselves with different (dynamic) IPv6 addresses under this static /48 prefix over time.
And I need to go to the LAN configuration page and specify that plus some 'xxxx' of my choosing and prefix length of 48 there so that it stays the way it is. That way the router WAN address may change (which I don't care about) but my server address will be fixed.
Yes, sounds about right. LAN settings should not involve the FEED prefix at all, only the /48 prefix. If you need to specify anything, just use the first available /64 of the /48, which will be 2a02:390:xxxx:
0::/64 (where xxxx is unique to your connection). Note you can shorten this to just 2a02:390:xxxx::/64 (removing the
0) under existing IPv6 notation, which is why it's handy to just use this very first /64 of the /48.