If the OSB is glued on rather than nailed / screwed, it does make the beam a lot easier to analyze, just like an I-beam as you had done.
A quick look at your calculations (or maybe I am not following them) shows that the box beam is adequate for the load as is? (Not included the girder portion of the beam.) If this is the case, the girder / diagonal framing will only further to stiffen this and make the beam perform better. Or, conversely, you could argue that you could take the diagonal framing out all together, and use the standard box beam.
Analyzing the girder portion could also be done in Excel or calculator using a matrix structural analysis technique. Essentially, the same way a finite element analysis or structural analysis software (including Visual Analysis or Robobat) would do it, except, you have to do most of the work (rather than the software.) The nice thing is though, is, it is free (assuming your time isn't worth anything...)
Once you analyzed the girder, you could determine an equivalent moment of inertia (I) and combine this "I" with the "I" component from the OSB.
An interesting project to analyze, and, a chance for me to talk engineerese with someone, which, is even cooler yet!
Joe