I would take the bathtub out, take it to a welding shop have them cut out the rusty part and weld in a new piece, then spray, bake in a kiln and when you put it back together put some clear silicone around it so water doesn’t infiltrate again
There's the catch. Only way to get the bathtub out means ripping out the tile and that means ripping out the three walls. So at that juncture the tub goes with the demo debris.
I didn't know this before, but bathtubs have about 3-4" of metal turned up and tiled over. That's how they keep water intrusion at bay.
If I could get the tub out easily, I'd toss it. The tubs installed in my house had defective coatings. One is holed all the way thru because it's not my tub, so it was out of sight out of mind till it was too late.
The one I'm looking at now is a guest bath, so other than dog baths hasn't been used much. After having one with a massive hole in it, I've decided to attack this one and try to save it. The one with the hole I am converting to a shower after demo'ing the walls, etc.