There is no problem, I am uncertain if I am approaching it correctly. In the video link the person states "don't model your welds, calculate your welds after." That is what I am doing and it makes perfect sense. The model I gave is just a test subject for the Fusion 360 simulation experimentation.
Say the load is 10,000lbs, do the inside braces need to be larger? Can they be smaller? Don't know but the simulation will guide me. Once satisfied, I find the stresses at those locations, you can then say you need "x" size weld at "y" length for the appropriate stress given in that area.
Is it better practice to simulate with it all bonded together or each piece individually with a calculator handy?