steel flexes. no way around that.
I have a cannondale t400. Huge aluminum tubing, cromo fork. the fork flexes enough and with 32mm tires its pretty soft on bumps. But it does eliminate the flexy bottom bracket. I don't think you will be able to find a stiff enough steel frame when your weight is over 180. The extra weight us clydes have causes the steel to flex more when we aren't even out of the saddle.
I can jiggle my 64cm SR semipro, which is steel. like 2 inches or so of lateral movement. just the nature of the beast (steel).