GMAW Flux core with gas shielding, (totally different than the fluxcore sold in 2 lb spools for hobby machines) is a fantastic wire, it can really lay down some metal, but to take full advantage, you do need to be able to run in spray transfer.
Transition to real spray transfer will happen at lower current with a FC wire than a solid wire, but figure 250 A output is good as a minimum machine to run that wire.
Also note most structural “dual shield” wires are not even available in less than a 30lb spools, and .035 dia. Hobart was selling one for the home/hobby market for a bit (think they called it 10 gauge or something confusing like that, which since they were aiming for auto body work didn’t help).
These wires are often formulated for a high volume industrial application and can be a little finicky in their parameters, but really shine for welding thicker material that has some surface contaminants (over 1/4, with mill scale -light rust).
Dot even consider using this class of wire without the recommended gas.
I don’t have a good general purpose mild steel fab wire recommendation, can anyone chime in with that?