Well, I got it working.
For some reason, setting the prefix hint to 0 fixed it.
I had tried this previously, so some other configuration change must have been necessary.
I do now have working IPv6 on my network now though - which is neat.
Thank you so much to everyone who got involved in helping me figure this one out! 
Nice to read that you have it now working …
I’m very curiouse as to the prefix size they actually gave you because a prefix hint to 0 in the IPv6 prefix field indicates that the requesting router has no preference for the prefix …
And prefix hint ::/0 is default … it’s not shown in @OPs first shown configuration so it’s been at default initially. So it must have been something other which made the difference.
They did allocate a /48 prefix as requested. Although with the prefix hint set to 48 they won’t allocate it. The prefix hint must be 0. Weird.
Yes that is weird and frankly makes no sense to me.
So does everyone get a /48 or must the subscriber request that on a per account basis via what method? ONT, eMail, phone, lettermail ???