Finally came around and did it!

The difficulty progression curve is now... functional. It might not be super well balanced yet, but that's okay, I intend on working on it as I test other things, to save some time.

What I did is this : At the start of every level, a point budget is prepared. Then, from that point budget, I "purchase" asteroids by spending points, and then I spend some more points on stats for each asteroid. This helps each play feel a bit different, but not 100% random, and always harder.

This system is also a great stepping stone to allow me to grant a score value to each asteroid : The faster they are, the more points they're worth!