ip6tables-translate -t nat -A postrouting -o eth0 -p tcp -j SNAT --to [fec0::1234]:80
nft add rule ip6 nat postrouting oifname "eth0" meta l4proto tcp counter snat to [fec0::1234]:80

ip6tables-translate -t nat -A postrouting -o eth0 -p tcp -j SNAT --to [fec0::1234]:1-20
nft add rule ip6 nat postrouting oifname "eth0" meta l4proto tcp counter snat to [fec0::1234]:1-20

ip6tables-translate -t nat -A postrouting -o eth0 -p tcp -j SNAT --to [fec0::1234]:123 --random
nft add rule ip6 nat postrouting oifname "eth0" meta l4proto tcp counter snat to [fec0::1234]:123 random

ip6tables-translate -t nat -A postrouting -o eth0 -p tcp -j SNAT --to [fec0::1234]:123 --random-fully --persistent
nft add rule ip6 nat postrouting oifname "eth0" meta l4proto tcp counter snat to [fec0::1234]:123 fully-random,persistent