Multi WAN both on DHCP

It’s not difficult, just pretend that you have static config, add everything you need, and then use dhcp lease script to update dynamic values. For example, you can have route like this:

/ip route
add comment=dhcp1 disabled=yes distance=1 gateway=1.2.3.4 routing-mark=wan1

And this simple lease script:

:if ($bound=1) do={
  /ip route set [find where comment="dhcp1"] gateway=$"gateway-address" disabled=no
} else={
  /ip route set [find where comment="dhcp1"] disabled=yes
}