Ipv6 - subnetting when I have a /64 PD

My ISP gives out ipv6 with a prefix delegation of /64. Is it possible to create multiple ipv6 /68 or /69 subnets on my network? If yes, then how?

I have the ipv4 subnets working to my satisfaction. However, only my main LAN subnet is fully working on ipv6 with internally routable addresses (eg. fda6:... ) and externally routable addresses (eg. 2401:... ). The other subnets only get locally routable addresses. I understand that this is likely that the router / odhcpd is fully adhering to specifications and refusing to create subnets of /68.

The ISP seems to be marketing driven rather than technically driven, providing mobile, fiber, and TV services. Therefore I see little chance of getting them to change.

Theoretically, yes - but you still don't want to go there. prefixes smaller than /64 are possible, however only with DHCPv6 or static assignments, while SLAAC does not work with those at all (hello android and other devices), aside from further fun with privacy extensions and many other problems.