Minimal Service Levels in Pool selections

You can try other options per my knowledge. Create two pool http_pool1 and http_poo2 with 5 members.

Now you can manipulate the traffic if primary pool(http_pool1) member fail 4/5 node.

Hope below iRule will work...

when HTTP_REQUEST {
  if { [active_members http_pool1] >= 4 } {
    pool http_pool1
  } else { 
    pool http_pool2
	}
}

Try and let us know..