What do you do with a 95% oxygen atmosphere with now only ~60 times the mass of Earth's atmopshere? You could make a small ocean out of it, but that's a huge amount of H to source.
That would be useful for about 1-2% of it; but it would be better to bring it to Mars as CO2 in the first place and crazy expensive. I like the idea, though. Maybe space habitats around Venus instead.