First human reproduction and birth (ok, maybe just birth, but ey….) in space. That will take crazy amount of advancements in radiation blocking, muscle mass preservation and bone development.

The next major milestone would be getting them back off of Mars.

We've had lots of practice leaving Earth, but nothing man made has ever launched back off the surface of Mars into orbit.

Establishment of a permanent lunar base / lunar space station, permanent research presence on Mars, then first manned mission to deep solar system bodies like Encaladus.

I think the next steps before putting people on Mars are developing Martian geology and determining a fuel source that can leave Martian orbit. Getting people on Mars is pointless if we can't bring anything back.

I think the first person on Mars should be close to retirement age and expect to stay and die there.

So the next achievement would be to get them an optional supply mission. And then for them to die after a few years, without too much discomfort, not from technical failure and without living through the effects of radiation.

crazy amount of advancements in radiation blocking, muscle mass preservation and bone development

I don't think that much advancement actually needs to be made. We know how to make effective radiation shielding. We know that gravity can be simulated using a rotating habitat. We just need launch costs to drop enough to make constructing such structures feasible on a realistic budget.