Sending spores to a planet that already has life might work. But I can’t help but think whatever life we introduce would be at a disadvantage. Maybe life on that planet never incorporated certain proteins, vitamins, or amino acids and whatever we send just ends up getting scurvy and dies out.
Moss has already adapted to barren environments. Its niche is growing where nothing else grows. Like, on top of rock. It's not having roots, not mingling with modern temptations in the soil. Most mosses actually aren't doing well in competitive, complex ecosystems full of nutrients and such.
A community of several different kinds of bacteria would have better chances than a single species, but for bacteria there is certainly no need for thousands of species.
Autotrophic bacteria would need only an environment providing less than 20 essential chemical elements (most of which belong to the most abundant elements, a notable exception being molybdenum) and either solar light for energy, neither too little nor too much, or a chemical source of energy, like dihydrogen + carbon dioxide, which can be provided by volcanic gases or by the reaction of water with volcanic rocks.
There would have been many places in the Solar System suitable for bacteria, except that where there is water, it is usually too cold, and where it is not too cold, there is no water.
* I am aware of various experiments that did attempt to raise animals in perfectly sterile environments, where they died, but the only way to sterilise and maintain sterility, are extream, and largely impossible while keeping any single lifeform, alive.ie: it is far from the default
But I have hard time believing even hardened organisms like moss or tardigrades could survive millions of years of hard vacuum and extreme cosmic radiation. Maybe embedded in some properly protective envelope, 1 out of billion trillion might. And then that one has 1 out of billion billion trillion chance to land eventually on a place that could be called livable. Or add few extra zeroes.
In genetal, nature works with small chances, look how many seeds a plant gives and how few of them will be a new plant.
(Or how many sperms are created for 1 human)
But sure, chances here are way, way lower.
When you come to some place and change it drastically, is it a good thing or a bad thing? I don't think it is. There are some excuses that I can accept, but if you do it "just for fun" of it, I think it is an evil deed.
Places have their own history, their own shapes and forms, and then someone comes and wipes it off just because they can. It cannot be Good, can't it?
(And terrorism is often more about causing fear than raw death counts.)