So if you say this works:
:if ( [/ping 8.8.8.8 interface= "ETHERT 2" count=6 ] = 0) do={/interface disable numbers=1}
Then the following should work if you add the comment label "ISP2" to the route going to your second provider. ETHERT 2 is not disabled this way, this because you can never detect when it is active again because it is disabled by you.
Down? then disable the route.
:if ( [/ping 8.8.8.8 interface= "ETHERT 2" count=6 ] = 0) do={/ip route disable [find comment=ISP2]}
Up? then enable the route again.
:if ( [/ping 8.8.8.8 interface= "ETHERT 2" count=6 ] != 0) do={/ip route enable [find comment=ISP2]}
This is really crude/simple/basic and I don't know if the traffic is routed through ISP1 when the label is not found/active in routing.