Yup...you seem to have a good plan on how to load out the platform...loading anything up high has negative outcomes beyond doing dead lifts putting the top up and down...plus weight is weight on the vehicle and I need to be very serious about what I want and what I need to heap on an already over-loaded Tundra..like you I have back wall steps and can access the back 1/3 of the top area for possible loading. Might consider your approach or build my own platform and forgo the towers and bars by screwing directly to the nuts in the Yak Tracks. Plus for us the Hawk as outfitted clears [solar panel is highest point] my 8' shop door by no more than 3/4" so putting bars up top would be a no-go.
The Rhino platform weighs about 25lbs but the 2 sets of Yak towers and bars are a heavy combo...consider using conduit to replace bars if it proves too much of a grunt ...this was a solution by a forum member..
300w on top...wow, serious solar..you are correct the panels are not light...one reason I have a portable plug in panel for the Hawk to compliment the roof mounted 160W.
Keep us posted on your project...good luck.
Phil
Ps...Just saw the square bars...nice touch and they may not weigh anywhere near what the Yak round bars weigh.