I have never tried oil based polyurethane on my airplanes because we when we had our floors done with it we had to do it in the summer with a lot of ventilation and we were warned it was not safe to breathe.
I'm surprised you would consider shellac worse, considering the carrier is just alcohol. Yes, it has a strong smell at first, but I found the smell dissipated quickly, as I stated. I think it's safer than oil based polyurethane and I don't think the odor lingers nearly as long.
Jim