What good is your IPv6, Mr Anderson, if your upstream provider and/or middleboxes along the way do not support it?

I'm happy to report that - to my utter amazement - my ISP's on-prem device does /64 prefix delegation to each (DHCPv6?) client.

Yes, right now every large provider does that, which is great. That was not the case when the first p2p networks were growing big (Napster, Gnutella, that kind of thing).

Can you request a bigger delegation? a single /64 is very limiting, since that effectively limits you to 1 subnet (so no extra guest / IoT subnet)

You could ask your ISP/RSP for another /64? My RSP can do so I believe, not that I've asked.

With "good" ISPs that support IPv6 well you'd only have to change your DHCPv6 config to request a larger delegation instead of the default /64.

Unfortunately, some ISPs (including mine) limit this to /64 only, which is extremely limiting.

It already works. Sometimes the cure is worse than the disease.