Installing Hammock Chair from Ceiling with Metal Joists
I think you probably ought to abandon the project at this point.  Metal joists can be surprisingly light gauge steel.  Sheet metal screws would not be strong and reliable enough for the pullout strength you need, especially holding a moving object.
Toggle bolts would probably be strong enough, I don't see the bolts ripping right out of the sheet metal, but I could see the sheet metal flexing enough to mess up the drywall and possibly deform the joist, which would be a very expensive repair.