The updated behavior of the Collector in EA 2 is a step in the right direction, but it does not sell its intelligence entirely; it still feels very much like a predictable video game creature. Instead of the Collector being so obvious and lumbering, have it be stealthy. This puts the player on edge the entire time they are in the Collector’s domain.
The Collector needs to be able to see the player from a long distance and sneak up. To achieve this, have the Collector swim horizontally with its face backwards at high speed to navigate the far field effectively (speed 3), much like a squid or cuttlefish, while it is in the far field and has detected the player. While in this state, the Collector should try to get behind the player while keeping its distance. Once it is behind the player, it turns to face them and moves slowly (not too slowly, as it has to be able to reach the player from the far field without detection / speed 2) towards them. This should be done silently and without boss music, so as to not arouse suspicion.
Once the Collector is in the near field (speed 1), it lights up and does its signature pulse animation, giving the player the opportunity to react—preferably in utter horror and fear. To add some variety, additionally have a small chance for it to be completely silent and have the PDA notify the player of an imminent leviathan presence.
All of this is in addition to its new walking behavior (speed 1), but allocate the walking behavior to pursuing the player while they try to escape the leviathan.
(speed 3 = fastest, speed 1 = slowest)