You should use the trailers gvw simply as a way to be safe. Historically a trailer owner will load up far more than they think as a new owner. So much stuff you don't think about, don't know about, don't know that you would like to have when you get to "that place". To calculate on anything else is simply fools play.
Now, you can control that...if you will, and, measure weights. Time consuming and a bit of an irritation. I doubt you can actually take a nice trip for 2 without 1200 lbs. but we've done this a while - and I'm sure others take much less; we used to head out for a long weekend with, I'm sure, much less than what you anticipate - but after a while it grows, and that is problematic with a minimal tow truck.
Personally, I like your approach, attitude and goal. Renting is a losing proposition on RVs any way you look at it IMO. Your truck, IMO, is a problem. The issues it has can be mitigated, somewhat, by what I mentioned previously; and things that you should do no matter what to make sure that you can do what you want to do with this trailer. I still think you are exceeding your limits.
I fully understand wanting to "test the waters" or using an RV. To buy one always requires a "match" with the TV. And, I really want you to be able to do that - RVing is a really fun activity.
As far as the truck always being garaged; that helps with rust but has nothing to do with the capability of the vehicle suspension, age and other things. As the truck sits things settle and become even less capable in a vehicle like the Tundra. A correlation, hopefully no one objects to, is the spring in a pistol magazine; I kept one filled with ammo for years for one of my weapons; didn't particularly like that gun so it sat.....for years. Decided it needed a workout and took it out; resulted in constant feeding failures. Cause? The spring in the mag had weakened due to just "sitting" with the pressure of the ammo in the mag. The truck has the same problems.
I truly want you to try an RV - they are really fun. I also really want you to get something that won't put you, yours and others in peril. The Japanese, to this day, do not understand towing large trailers - they don't do that. They have made huge strides in trying to make a truck that "fits" the American market...and still haven't in the towing sector. Yours is from years ago so has marginal, at best, towing capability when it comes to a 6800 lb. trailer.
Reinforce the truck; check everything out, keep those weights way down and, hopefully, have fun, but, optimally......buy a smaller trailer...or a larger truck.
Edit: Just a thought; "I wanna be rich - so I can buy a new truck"...…..I hope, and I'm sure you know, that just "wanting" to be rich won't get you there. Hopefully you are taking the actions/making the decisions that are required to get there so there will never be another "I wanna be" in your comments.
Here's hoping you reach that goal.