Looks very nice in fact!
Adding another T200 aft is a good idea especially when the tether i mounted in the rear as that tether creates a counter force when descending. An extra T200 to push down and balance will be useful and also like you say for increased lifting capabilities.
I do agree to tilt the vertical thrusters as it looks like some of the flow will counteract inside the frame (in the front)
I see you still keep the original buoyancy so I guess the distance between Cog and CoB is narrow making the vehicle unstable (off course you have thrusters to counteract this now) Adding more buoyancy on the top and led under the bottom would allow the Vertical thrusters to work less = draw less current) Doing so might let you save the cost and power consumption of installing that extra thruster?
Have you considered to move the two aft horizontal thrusters further aft to be in line with the aft vertical thruster? The reason why I sketched only one single vertical rear thruster was so that the aft horizontal thrusters could be moved aft, making turns smoother, ROV less responsive, and at the same time add momentum to it. My tether is connected at the top, lifting in the center. The two fwd vertical thrusters will counteract for the “nose up” when going horizontally during survey. Also why i want the frame to be longer and heavier so that the CoG can do its part of counteracting the “nose up” tilting.
Is it at all possible to open the side frames a little more as it seems to be some enlarged projected area there. Something like this?
If the plastic get week you may consider to stiffening it with sheet aluminium on the the backside?
All in all i like the ROV and how much room you have to add on inside the frame. Its a slick frame unlikely to snag on anything.