RouterOS v7 supports DHCPv6 prefix delegation. You can request a delegated /64 per downstream interface and announce itself as router using these prefixes. You can still use your MikroTik device as router, stateful firewall, proxy. You don't have to mess with smaller than /64 allocations on links unless your provider forces a broken CPE on you that doesn't support DHCPv6 prefix delegation.

Have you actually seen any large scale deployments of CPEs without an active IPv6 firewall blocking incoming connections by default?