Flange support

Hi, I've redone my upstairs small half bath (repainted,fixed subfloor,installed new flooring and vinyl base board) now I see that the flange is not supported and there is movement there when I put pressure on it.
My question is: Is there a way to secure the flange without ripping up all the work I've done? It is level with the vinyl floor and I was hoping there is something that I can put under the edge and screw to the floor hiding it under the toilet.
Also the old flange is quite rusty, how do I fix this grind down to bare metal? or something else?
Thanks, Gary

The sub floor and finished floor should be under the flange and the flange screwed through the finished floor and into the sub floor. The flange is supposed to set on top of the finished floor. You can not get by with a flange that moves.

If you poke a hole in the ceiling below, you could probably cobble up something to anchor the flange. Maybe a plywood patch, with a clearance hole then cut in half to fit around the pipe. If you held it in place while someone screwed down from the top, you'd sandwich the existing floor. You might want to put some construction adhesive on it to make it stronger. Or, maybe a couple of 2x4 lengths that you could screw into through the flange could do the same thing. easier to patch the ceiling than tear up the floor.

There are repair flanges that sometimes have a wider spacing of the screw holes that might catch into something solid.

If the flange can move, you won't get a good seal, and basically, the flange is what holds the toilet down and in position as well as creating the seal. You don't want a flange that moves, as then your toilet will, too.