This is stud walls on top of a concrete slab. @ feet on center. 2x4
trusses 4 feet apart. The truss manufacture says no drywall or osb
ceiling due to the weight. Apparently they were designed for a "pole
barn" style. (No interrior ceiling). The lightest thing I could think
of was soffit or suspended.

Could you add your own ceiling joists "between" the trusses to support a
drywall ceiling? Assuming you won't be storing anything in the attic, all
you need is enough strength to support the drywall.

2x10's at 16" OC should work for a 30' span, though you might have a hard
time finding 30' lumber (a lumber yard might be able to special order and
deliver). Wood I-joists might be easier to find in those lengths, and you
might be able to use a smaller size (check with the I-joist manufacturer).

Of course, the extra framing might add another $500-800 to the project, so
you would have to decide how important a ceiling is.

Just a thought...

Anthony