Hi Ragger! I finished RF Level 3 roughly five years ago in preparation for a trip to Brussels and Paris, and at the end of it, I was evaluated at B1. Just to frame context, this testing happened after I returned from my trip. I could get by as a tourist, but I had a hard time understanding and expressing myself solely in French. So after my trip, I found an online tutor who suggested two classes (1 hr. each) per week, where he would choose material to read and discuss, then would correct my errors and introduce vocabulary (usually something colloquial). I'm still doing this ritual, just for practice, in addition to going to cities in France every year. So, to answer your questions.
1. The confidence was not always a straight line for me. Every time I came back from France, there was always some little encounter or something that made me realize that I still had to work on the oral comprehension. That said, I'd say that I didn't feel confident until about three years ago (i.e. two years of "post Rocket" independent French study).
2. In addition to Rocket, I strongly suggesting working with a grammar books like Grammaire Progressive du Français alongside it because the Rocket series is more about getting you speaking from the onset, and the intricacies of grammar may need more explanation/context to grasp.
As an after thought, I think that working with a tutor right from the start along Rocket is a good idea (although I tried this method with Japanese, and found that I wasn't really using the application as much).
3. For additional learning, again, it depends on why you're learning French. For me, it was all about comprehension, so most of what I do now is how to improve it. The biggest obstacle for me is that people on the street don't always speak textbook French, so the hurdle for me is understanding colloquialisms, how people pronounce things in real life (like the 'e' and 'ne' that's jumped). I used to do dictées a lot with teachers. It's a good practice of comprehension and reinforcing that you understand your grammar tenses because even though they sound similar, they are written differently. Now, I do transcription exercises.
I also read a lot of books in French, but this is more of a personal thing because I like the precision of words, and I think reading is the best way to really expand and add nuance to your vocabulary.
4. Again, I would recommend an online tutor, or you can also do language exchanges. It's the best way to overcome that fear of speaking in another language.
C'est tout pour l'instant ! Bon courage !