This needs selective NATTING on the server for the ULA addresses:
https://openwrt.org/docs/guide-user/firewall/fw3_configurations/fw3_nat#selective_nat
config nat 'nat6'
option family 'ipv6'
option proto 'all'
option src 'wan'
option src_ip 'fd2a:b5d8:7ec0:9::/64'
option target 'MASQUERADE'
but the way you are doing seems to amount to the same 
See also : How to setup Wireguard with IPv6-PD? - #21 by egc