Considering the realities and complexities of launching something like Starliner, it would be trivially easy to simply prevent him from launching.
Yeah id say 8-10 is a bit more optimistic. They still have to find a suitable site, get their infrastructure there and make sure its all working then launch a crewed mission.
I could see a flyby and say a landing on Deimos/Phobos in 4. That would be significantly easier.
Deimos/Phobos are basically just asteroids caught by the gravity of Mars and it is theorized that they aren't even all that solid due to the gravitational forces at work on them - it would actually be 100x harder to land something on them than Mars if you launched it from Earth.
Time table wise with Starship, you have to
a) Get it reliably working for Earth launches - realistically I think we are 2+ years from manned launches bare minimum
b) They still have to create a lander Starship for the moon mission (depends on point 'a') that can land on the moon with no supporting infrastructure AND takeoff
c) Moon lander will give some some working knowledge they can apply towards a Mars mission
d) Assuming Earth + Moon stuff is squared away, you're probably looking at 7ish years before they even attempt a Mars unmanned landing (NASA will also be influencing this a lot, which would potentially slow things down).
e) Assuming the unmanned attempt goes off without a hitch (you'd have to wait around 8-9 months to find out), then you'd want to make supply runs assuming you find an ideal human landing spot. Otherwise that has to be identified.
f) You'd probably need 5-10 supply ship runs minimum - if they launch multiple at sametime or in close proximity, then that will speed things up. But assume at bare minimum at least 1.5-2.5 years to supply a spot. A first mission is going to have to massively oversupply with stuff - this is all unexplored territory and you'd want triple redundancies on anything astronauts would need to survive
g) Assuming all that goes off fine, then maybe within a year of the last supply launch you send humans
I think one of the biggest hurdles is going to be finding the ideal long term habitation area. They can always "rough it" for the 1st mission, but subsequent missions will probably want to locate somewhere that reduces costs (water ice access, caves/lava tubes to allow for larger habitations protected from radiation, proximity with possible scientific sites of interest)