1- “ Parasitic life on earth (viruses, bacteria, etc.) are very very unlikely to be harmful to an alien life with an alien biology.” We don’t know that. We might suspect that, but we don’t know that. Insufficient data, and all that. That being said, viruses and bacteria evolve at crazy speeds compared to the rest. Just sayin’.
2 — given the ever-increasing trajectory of technological change from even the 1950’s until now, and given the likely increase of rate of change in the coming decades and generations, we cannot know the limits of technology…nor can we know whether advanced civilizations even want to spread. If such a race were a million years old, then it would be 1/1350th of the age of the universe. Like I said, a flash in the pan.