(There is no DHCP6 server configuration as there are no sub-routers on LAN3)

#/etc/systemd/network/enp0s3.network
#
[Match]
Name=enp0s3

[Network]
LinkLocalAddressing=ipv6
# SLAAC IPv6 for obtaining the default route
IPv6AcceptRA=yes
DHCP=yes
IPForward=yes

[DHCPv4]
Hostname=net2
UseHostname=no
# Ignore the hostname from the DHCP server
UseDNS=yes
UseNTP=yes
UseRoutes=yes
UseGateway=yes

[IPv6AcceptRA]
UseDNS=yes
DHCPv6Client=yes

[IPv6PrefixDelegation]
Managed=yes

[DHCPv6]
ForceDHCPv6PDOtherInformation=yes
UseHostname=no
UseDNS=yes
UseNTP=yes

#/etc/systemd/network/enp0s8.network
#
[Match]
Name=enp0s8

[Network]
Description=Link to my secondary LAN. Ref: see: enp0s3.network
DHCPv6PrefixDelegation=yes
IPv6AcceptRA=no
IPv6SendRA=yes
DHCPServer=yes

[IPv6SendRA]
Managed=yes
RouterLifetimeSec=1200

[DHCPPrefixDelegation]
#SubnetId=0x11

[Address]
Address=192.168.3.1/24

[DHCPServer]
PoolOffset=200
PoolSize=10
EmitDNS=yes

[DHCPServerStaticLease]
MACAddress=08:00:27:43:3c:53
Address=192.168.3.100