Re-delegate delegated prefix

[Alphaphi-by]

Hi all,

from my ISP I get a /56 prefix, out of which I can cut 16 /60 prefixes. I would like to delegate two of these to two downstream routers, so that they again can build two or three subnets each.

Unfortunately the ISP's prefix changes every now and then, which means I cannot do a static setup. Instead I need to track the WAN interface for changes.

Here is a diagram:

                                Internet-Provider
                                       ^   
                                       |WAN
         +-----------------------------+----------------------------+
         |                       PREFIX:fb00::/56                   |   
         |                                                          |   
         |                           pfSense                        |   
         |                                                          |   
         | tracked            tracked            tracked            |   
         | interface: WAN     interface: WAN     interface: WAN     |   
         | IPv6 prefix ID: 10 IPv6 prefix ID: 20 IPv6 prefix ID: 30 |
         |                                                          |   
         +----------------------------------------------------------+
                   | igb0               | igb1         | igb2
                   |                    |              +------+
                   |                    |                     |   
                   |                    |                     |   
         +------------------+  +------------------+  +------------------+
         | downstream       |  | downstream       |  | LAN              |   
         | router 1         |  | router 2         |  |                  |
         | PREFIX:fb10::/60 |  | PREFIX:fb20::/60 |  | PREFIX:fb30::/64 |
         +------------------+  +------------------+  +------------------+
             |          |           |          |              |
             v          v           v          v              v
          clients    clients      clients    clients        clients
         :fb11::/64 :fb12::/64   :fb21::/64 :fb22::/64     :fb30::/64

I've been reading a lot, and spent a long time today to find a working configuration, to no avail. It looks to me as if this setup is not possible. I can only pick one subnet out of the 256 possible subnets a /56 provides, and assign it to a interface.
I'm surprised, because the whole point about prefix delegation is that you can cut a big chunk into pieces, and re-delegate again, which is what providers are doing. So why isn't it possible to re-delegate a smaller portion of what I get from the provider?

Am I missing or misunderstand something?

Any ideas and hints are much appreciated.