Another possible strategy (I play conventional starts), which must be played from the beginning, is to combine civilian mining and regular colonies.
Initially, I only survey bodies that are good for colonization: Luna, Mars, Mercury, and possibly some of the larger gas giant moons.
At game start, I immediately start enlarging my naval shipyard and when it's big enough (roughly 3000 tons), I build a survey vessel with conventional rockets, conventional geosurvey equipment and a tiny cargo space. Slow and primitive but adequate for this task.
I then survey those few bodies, and if any has the potential to spawn a civilian mining operation, I leave it untouched and dump 2 infrastructure on an empty body. This unlocks the creation of civilian mining. Then I push my wealth income as high as feasible with financial centers and accompanying research, to increase the chance of triggering a civilian mining center. Once it is in place, which is why I initially restrict my initial surveys to places I want the mining colonies to spawn, I add a regular colony to it.
If you build a regular colony, no new Civilian mining centers can spawn. But once the first civilian mining center is in place, it will expand over time, adding up to the equivalent of hundreds of automines. Plus you get a free military unit to keep order.
Civilian mining centers spawn if the body contains Duranium or Gallicite, minimum 10,000 units with concentration 0.7 or more. Higher wealth income increases the spawn chance.