A lot of care is put into making sure we don't track Earth microbes to Mars in the first place to make sure if we do find life we'll be pretty sure it's not just a hitchhiker flown in from Earth in the last 40 years of space exploration.

That's historically been true, but won't always be the case. NASA has talked about putting humans on Mars within a couple of decades. It may be sensible to try putting other living things on Mars in advance of humanity...

For a long time this will still be the case. Until we're going to be putting humans on Mars (at which point there are very limited options for continuing the sterility, because we're lousy with them) avoiding contamination will be a pretty high goal for missions.

This! Having supplies/environment primed and ready when humans arrive could make the difference between a thriving colony and a miserable death.

The first colonies won't get any help from microbes on the surface of Mars, it's too cold and dry for much of anything useful from Earth to survive much less thrive. And the timescales for anything that could a) thrive and b) have a measurable impact on the lives of colonist are way longer than (I hope) any mission will land since it'd take decades/centuries for there to be a useful impact. Eventually yeah introducing life to Mars will be a useful prospect but we're talking about kickstarting a whole planet's ecosystem not something to do haphazardly or that will show large impacts in a single lifetime.

All we'd need is a bubble-greenhouse to be growing some wheat or something - don't have to terraform the entire place.

They would still have to live through the 6+ months of hard space before making it to Mars. So you'd also have to include a heater and a window, at a minimum. Also, it would need to be a biosphere, to generate carbon dioxide until it reached Mars.

Weight, though...