I don't believe simply sending any kind of (micro)organisms would succeed in turning Mars to a livable planet similar to Earth.
Truly and realistically it requires much more.
We need to have the necessary technologies first and even then it could take hundreds of years.
First needed tech is a small (nuclear) power generator that can produce a lot of power.
Something almost like the fictional Arc Reactor in Iron Man/Marvel movies.
(Of course if cheap enough that kind of power generator would allow a lot more for humanity like turning all deserts to farmland
(because it would allow producing a lot of drinkable water from seas/oceans and cheaply pumping it anywhere with pipelines),
or it could be used to produce a lot of really cheap titanium, aluminum, carbon-fiber etc and make all kinds vehicles, buildings, even roads from them.)
Second needed tech is a pure electric drive for space probes, satellites, vehicles.
It must not be using any kind of fuel.
I think this maybe possible by creating giant dynamic electric/magnetic fields around the space vehicles.
Because space is always filled with charged particles, ions flying around.
If a space vehicle can create giant electric/magnetic fields and apply their force(s) to the charged particles/ions flying around
then it should get a counter force applied on the vehicle itself.
(So for example if the charged particles/ions are pushed towards to the rear of the vehicle that would create a force pushing the vehicle forward.)
(This kind of space drive should also reduce radiation damage on space vehicles.)
(The field would probably need to be dynamic. Imagine how propeller of a ship/plane works, or arms of a swimmer, or even motions of a snake etc.)
Now assume we have these two technologies (in the future).
Assume we first created thousands(?) of probes and sent them everywhere in the solar system.
So that we can track all planets, moons, large asteroids, comets in the solar system in real time.
Imagine we also created a computer simulation of the whole solar system by using the tracking data we have.
That would allow us to predict both short and long term changes in the solar system.
We could see what would happen if we changed the orbit of a large asteroid/comet and made it collide with Mars.
Imagine we started choosing larger and larger asteroids/comets and sent lots of probes to them.
Those probes later attached themselves to their target objects and started slowly changing their orbits
so that eventually each would enter into a collision course with Mars.
If we could do that then overtime we could increase mass of Mars and maybe even bring it closer to sun.
(Comets would also provide lots of water for Mars.)
(I think it should be possible to simulate today, one by one changing orbits of asteroids/comets and colliding them with Mars
and watch how it would effect the stability of solar system in short/long term.)
Not just for Mars, but for all kinds of human space exploration activities there is still one more critical tech needed.
A true oxygen generator.
Keep carrying tons of water or other chemicals to space just to generate oxygen for people cannot be practical for long term.
There is a huge need for a device that can filter CO2 from air, separate it into oxygen and carbon, release back oxygen to the air
and by using only electricity, nothing else.