IPv6 address from pool chaging

Hi,
I have an issue with IPv6 addressing. When I get a /56 prefix from my ISP through DHCP-PD, I cannot statically set /64 address prefixes to bridges, as they change from time to time. For example:
I get a prefix like this: 2a00:xxxx:xxxx:1700/56
I assign addresses like this to bridges: 2a00:xxxx:xxxx:1701::1/64, 2a00:xxxx:xxxx:1702::1/64
They are also assigned to PPPoE users so I have to get them from the pool.

2a00:xxxx:xxxx:1701::1/64 ← I want the bold part to be static

Is there a way to set them statically, but make them still show up in the pool, because PPPoE needs to know if they are available?

Kind regards,
Luka