Is it really an invasive species or rather opportunistic?

I would call it opportunistic

What are the conditions that makes the thistle thrive?

They are probably growing in not that good soil, right? Hence the opportunity they seize to make themselves home.

Does it attempt to improve the soil and once it has reached its goal makes space for other species?

I'd guess they do, I have a few thistles growing on my soils as well. The soil here is in very bad shape and I welcome these deeply-rooted species.