I have experience with this. PPPoE over IPv6 via SLAAC aka true IPv6 aka stateless aka your ISP did their IPv6 101 right.
Run the DHCPv6 client to request prefix, add that to a pool, then advertise a /64 in IPv6>Address on a per-interface/VLAN basis (since you get a true /48 unlike my useless ISP with idiots in their NOC) from the /48 pool, I'm sure you can do the IPv6 math on this. Then in IPv6>ND, you will need to create ND for separate VLANs/Interface, that's it.
You will now have a true end-to-end IPv6 principle on all your VLANs/interfaces, with proper RFC compliant stateless IPv6 for all your interfaces.
For the "WAN" interface, IPv6 doesn't work like that, it's not IPv4. If you want to give the router itself an IPv6 address that's routable from the pool, you go to IPv6 Address and inside the prefix, use the "::1/64" notation, as for interface select "bridge" or whatever interface the router's RFC1918 main IP address is located in. That's it. It will show you the currently allocated IPv6 address with the notation, you can ping from external sources and it will reach your router.
RouterOS fully supports IPv6 stateless and stateful, excluding NPTv6/NAT66.
I envy you dude, a true proper IPv6 from your ISP. Unlike mine, I dropped IPv6 from my setup because on top of a stupid /64, my ISP decided to block ICMPv6 + break MTU/packet fragmentation among other things.
If you still have issues you can reach me on Telegram:
https://t.me/dark_nate