Akshually akshually atmospheric escape is a very slow process and would take even a large atmosphere thousands or maybe even millions of years to disappear. You could add the atmosphere first and have plenty of time to figure out how to keep it second.
The atmosphere would still be a CO2 toxic hellscape, so we could only send billionaires there for a long time.
What if the second solution is some technology - godlike to people from our time - that could start spinning the core of Mars to create a magnetosphere? That might be more difficult with people already living there.
I’ve seen people restart core spinning on earth with nukes, might work on Mars too!
https://m.imdb.com/title/tt0298814/?ref_=nv_sr_srsg_0_tt_2_nm_6_in_0_q_core