Yeah, the lack of rigidity with thin sheet metal might look and act cheesy.
Utilizing steel decking might be a cool way to go for that industrial look. The nature of the flutes would allow for easy "grasping" of the magnetic postings as opposed to a flat sheet.
My first thought is to run the decking horizontally.
Rigidity is built in.
Pick a profile that appeals....here's an example.



Some strategically placed Nelson Studs would be easily doable for more of that industrial effect and could be used to hang larger postings.

Utilize the wide flange (I-beam) posts for the vertical frame and use angle for the horizontal frame. Cope angle to fit wide flange.
No pan deck welds would have to be visible.

For seismic considerations :
If concrete deck, Simpson Titen bolts are excellent for the base plates.
Top will depend on backing system. Could be lags, Simpson SDS, or through bolts.