SMRs don't solve the cooling problem with nuclear reactors. They're still producing megawatts of thermal energy that needs to be dissipated. Mars isn't known for its flowing rivers or deep cerulean oceans. So using massive amounts of water to carry away heat (like we do on Earth) Isn't an option for cooling. It's atmosphere is far less dense then Earth's so air cooling a reactor would take vast fields of radiators.

Burying a reactor underground is not a cooling solution. Rock is not a great heat conductor. So the reactor will stay cool for a little while, to the heat capacity of the surrounding rock, and then melt because the rock isn't effectively carrying heat away from the reactor.

But not to worry we'll launch them on fantasy rockets! So long as we make the rockets out of Impracticalium rather than Unobtanium we should be fine. We can start tomorrow.

It's entirely possible to conceptualize ways a Mars/Moon/orbital colony could work. The issue isn't coming up with ideas on paper it's the actual implementation of those ideas that is actually difficult. Handwaving implementation details and actual engineering challenges is just writing science fiction. It might be interesting and entertaining but it's not actionable.