Found it! This was set to 0, and setting it to 1 fixed it:
# uci show glipv6
glipv6.globals=globals
glipv6.globals.enabled='1'
This allowed it to get a prefix via DHCPv6. It wasn’t assigning it to LAN, but after I changed the requested prefix from /60 to /64 on both LAN and GUEST (despite being unused), it worked, and everything has IPv6 now.
All fixed!