You are putting bottom runners on those doors,this should prevent sagging over time.
I made a folding door arrangement (4 doors) across a balcony using Johnson folding door hardware,Header above was 2 2 by 4's.
It has been in place about 5 years with no sagging,doors were fire rated and the four together weighed 140 pounds.
Probably shouldn't have asked my wife to lift them into place,but they wanted the vote!!! LOL
I appreciate your advice. I've decided to stick with the box beam idea.
In order to get the depth I need with lumber stood vertically, I'd have to rip plywood shims, etc...
I like the idea of all 2x6 lumber it will simplify the construction. I'd probably use a couple of flat 2x6s if I was using bi-folds as the weight really pivots from the very corners of the header and will never bear on the middle. With these sliding doors, it's feasible to have all 3 doors hanging from the middle of this beam.
It doesn't appear to me that these bottom guides have any intention of supporting the weight of the door at any point.