In truth, the best possible thing to do is take it all to a scale and weigh it.
There is some argument, but in my opinion, if you're good on the GCWR and all three GAWR, that's the most important part(s) to the equation. Even Toyota in some of their literature say the same. So you can do the math all day, but the end is the actual weight
Remember, these ratings are for any kind of trailer, and that includes tall TTs as well as small pop-ups, even though the towing characteristics are very different between the two. As pointed out, stopping is more important than moving. I can say that my experiences with short stops has been quite favorable: I've stopped my combined TV + TM on a rainy road in less space/time than the SUVs around me, and it seems to be able to stop roughly like the unburdened TV, which at least tells me the brakes are functioning properly
I know I can stop the whole combo with TM brakes alone, but I'm always on the flats. I do not know how it would fare on mountains or at altitude, but I have no plans to do so right now, so it's not a consideration.