Setting up multiple IPv6 routes over a single interface on Linux

Question

tldr; A LAN device is able to get 2 separate internet connection (1 NATted from ISP Router, 1 from manual PPP connection). Able to separate ISP-WAN and PPP-WAN IPv4 into 2 routes using ip rules but not IPv6.

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Apologies for the lengthy post. My requirement is to have 2 separate routing tables to connect to the internet through 2 separate routes.

I will be sharing how I am managing this with IPv4 - however IPv6 is where I am unable to figure out the steps.

I have a rather unique setup where I am able to get a global IPv6 address from my ISP in 2 ways.
My ISP provides an FTTH modem+router (RT) that uses PPPoE to connect to internet.

My LAN is on 192.168.5.0/24 and the ISP RT is the default Gateway at 192.168.5.1

I have a raspberry Pi RPi connected to RT using single interface br0 at IP 192.168.5.22 which is where I am performing this setup.

RT has an option to enable PPPoE passthrough that lets my Raspberry Pi also request for a PPP connection and hence,

1 - I can get a public IP on ppp0 for direct Internet access
2 - I can also access the internet through gateway 192.168.5.1 of RT

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Working IPv4 Setup

<WAN1> <WAN2>
    |    |
    |    `-RT - <PPP Public IP 1>
    |      `-br_lan - 192.168.5.1/24
    |        `-Laptop
    |          `-eth0 - 192.168.5.100/24 (default gateway: 192.168.5.1)
    |        `-RPi 
    |          `-eth0 - 192.168.5.22/24 (default gateway: 192.168.5.1 in table 111)
    `------------`-ppp0 - <PPP PUBLIC IP 2> (default gateway in table `main`)

ppp0 interface is the default gateway on RPi's main routing table:

# ip route show
default dev ppp0 scope link
192.168.5.0/24 dev br0 proto dhcp scope link src 192.168.5.22 metric 204
<redacted hop ip> dev ppp0 proto kernel scope link src <redacted PPP-WAN ip>

A new separate routing table contains the route to ISP-WAN with RT as gateway

# ip route show table 111
default via 192.168.5.1 dev br0 proto static
192.168.5.0/24 dev br0 proto static

A rule is set for all incoming connections from LAN to use ISP-WAN gateway

# ip rule show
0:      from all lookup local
0:      from 192.168.5.0/24 lookup 111

curl -4 ifconfig.co from RPi does return PPP-WAN public IP ✅ Any device on LAN using RPi as gateway gets the ISP-WAN public IP ✅

All traffic originating from Pi will use ppp0 and if needed I can add new rules to use ISP-WAN for any VPN interfaces :)
The corresponding systemd-networkd config:

# cat 11-br0.network
[Match]
Name=br0

[Network]
#DHCP=ipv6
DHCP=no
Address=192.168.5.22/24
Address=fdXX:22/64
DNS=1.1.1.1
#IPv6AcceptRA=yes
#IPv6PrivacyExtensions=true
IPv6AcceptRA=no

# Seperate LAN and PPP traffic in different tables
[Route]
Destination=0.0.0.0/0
Gateway=192.168.5.1
Table=111

[Route]
Destination=192.168.5.0/24
Table=111

[RoutingPolicyRule]
From=192.168.5.0/24
Table=111

[Link]
MACAddress=HH:MM:XX:YY:ZZ

The IPv6 part

There are 2 ways I can get IPv6 Access on RPi:
1. Through DHCPv6 requests on ppp0 with help of wide-dhcpv6-client.service (Ask Ubuntu link that describes this method) /etc/wide-dhcpv6/dhcp6c.conf:

# Default dhpc6c configuration: it assumes the address is autoconfigured using
# router advertisements.

profile default
{
  information-only;

  request domain-name-servers;
  request domain-name;

  script "/etc/wide-dhcpv6/dhcp6c-script";
};

interface ppp0 {
    # Request Prefix Delegation on ppp0, and give the received prefix id 0
    send ia-pd 0;
};

id-assoc pd 0 {
    prefix-interface br0 {
        # Assign subnet 1 to br0
        sla-len 0;
        sla-id 1;
        ifid-random;
    };
};

Resulting ip addr & routing table:

4: br0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/ether xx:xx:xx:xx:xx:xx brd ff:ff:ff:ff:ff:ff
    inet 192.168.5.22/24 brd 192.168.5.255 scope global br0
       valid_lft forever preferred_lft forever
#Global Public
    inet6 2401::XX/64 scope global
       valid_lft forever preferred_lft forever
#ULA
    inet6 fdXX::Y/64 scope global
       valid_lft forever preferred_lft forever
#Link local
    inet6 fe80::Y/64 scope link
       valid_lft forever preferred_lft forever

5: ppp0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1492 qdisc pfifo_fast state UNKNOWN group default qlen 3
    link/ppp
    inet <Public IPv4> peer <IPv4>/32 scope global ppp0
       valid_lft forever preferred_lft forever
    inet6 fe80::<ppp0 link local> peer fe80::123:46ff:fe07:0508/128 scope link
       valid_lft forever preferred_lft forever

# Route
::1 dev lo proto kernel metric 256 pref medium
<PPP IPv6 Prefix>::/64 dev br0 proto kernel metric 256 pref medium
fdXX:22::/64 dev br0 proto kernel metric 256 pref medium
fe80::123:46ff:fe07:0508 dev ppp0 proto kernel metric 256 pref medium
fe80::<ppp0 link local> dev ppp0 proto kernel metric 256 pref medium
fe80::/64 dev br0 proto kernel metric 256 pref medium
default via fe80::123:46ff:fe07:0508 dev ppp0 proto ra metric 1024 expires 1616sec hoplimit 64 pref medium

With this, anything from or through RPi (like VPN clients) will go out through ppp0 using the WAN prefix provided by PPP

2. Through RT's Router Advertisements on br0 using following change in [Network]:

# /etc/systemd/network/11-br0.network
[Network]
DHCP=ipv6
DNS=1.1.1.1
IPv6AcceptRA=yes
IPv6PrivacyExtensions=true

With this, all traffic from LAN as well as RPi both will use RT's ISP-WANv6 prefix.

The problem

If I enable any of the 2 methods, I cannot enable the other.
If I am using ISP-WAN, and I try to get an IPv6 from ppp0, wide-dhcpv6-client.service will throw client6_init: bind: Address already in use.

I tried to setup separate routing table for LAN br0 by a rule that routes all traffic from RT's ULA use the new table:

# Setting a static ULA + using DHCP to get Prefix from `RT`
[Network]
DHCP=ipv6
Address=fdZZ::1/64
IPv6AcceptRA=yes
IPv6PrivacyExtensions=true

# Route all outgoing traffic through the gateway provided by RT's RA
[Route]
Destination=::/0
Gateway=_ipv6ra
Table=111

# All traffic to LAN devices that use RT's ULA 
[Route]
Destination=fdXX::/64
Table=111

# All LAN devices (which use RT's ULA) will use the above routes configured in table 111
[RoutingPolicyRule]
From=fdXX::/64
Table=111

Other than the link local address fe80:: I don't get which address is in use. Tried removing teh link-local address from br0 but no luck.

The Global Prefix from ISP-WANv6 and ppp0 are both different, The ULA Prefix from RT is also different than the static ULA I configured manually.

In IPv4 I can easily create a separate route with a different default gateway, but unable to figure the equivalent in IPv6.

I'd need to be able to have 2 paths to the IPv6 internet, albeit using separate tables
And a rule that tells which route to take based on the originating source.
Any help would be greatly appreciated!

Edit: Since I am not sure this is the right place (I am not actually performing any server level stuff) I will be reposting this in Superuser and any solution I find there will be commented here too