If the colonization strategy is the Moon then Mars, I expect humanity would have the technology needed to colonize Mars easily while terraforming occurs.
The problem with an O'Neil Cylinder is bringing up enough processed material to build one.
I don't see the application of an O’Neil Cylinder within the Earth and Mars gravity wells given how expensive they would be to build next to better places to grow crops.
If one does get built, I would expect it in orbit around Jupiter or Saturn to support activity there.
If the colonization strategy is the Moon then Mars, I expect humanity would have the technology needed to colonize Mars easily while terraforming occurs.
The problem with an O'Neil Cylinder is bringing up enough processed material to build one.
One possible solution is a moon base. The moon is full of titanium and iron.
And then you could launch the stuff out of a weaker gravity well with no air resistance.
I don't see the application of an O’Neil Cylinder within the Earth and Mars gravity wells given how expensive they would be to build next to better places to grow crops.
If one does get built, I would expect it in orbit around Jupiter or Saturn to support activity there.