Sure making some seed bombs and throwing them at planets might work, but it will take a long time.
Realistically, Mars or the moon is where we'd be going first. An aquaponics setup would make the most logical sense for a starter colony to provide fish, oxygen and fuel(algae), radiation protection, recycling of urine, etc... With composting of human fecal matter through concentrated solar to produce biochar and hot water. Then usage of dust, pebbles, rocks, from digging out habitats, some for concrete, some for drainage, some for soil(biochar and compost) amendment.
All of which is doable right now and since life tends to always find a way, you'll get your specialized bacteria soon enough.