Hi Adam,
I'm a working bassist (mostly american roots music and theater work) that has never had a proper orchestral lesson/teaching in my life until last week. Originally I had bought, and planned, on studying out of Simandl Vol I. But, over the years I've heard so many good, and bad, things regarding this method, so I had my doubts. (I should add that my personally goals for my playing are not to be in an orchestra per se, but to sharpen my bowing/soloing skills and know the fingerboard very well, especially thumb position.)
My teacher (who just so happens to be a friend and knows my playing/learning curve pretty well) subtly showed enthusiasm when I mentioned Rabbath's Vol I. Me personally, I've been reading as much as I could find about Francois, watched everything there is to watch of him on youtube, gasped at the prices of his books, dvd's, cds etc. the past months.... basically, I'm a Francois Rabbath FAN NERD!

His approach, fingering ideology and bow technique amaze me. He seems like a very laid back teacher as well. Real. Not militant. I am a fan especially of the different fingerings and 'less' positions on the fingerboard.
Well, turns out my teacher is also a Rabbath fan and highly suggested his books for me and my personal playing goals.....but, he left the decision up to me on what book to keep studying out of. Honestly, the strict discipline of Simandl left hand technique scared me a bit. So, I bought Rabbath's Vol I, started into it on my own yesterday. Yes, it's all first position.... but I need that! Both for getting my fingering going in the right direction, and for strengthening my (weak) bow technique. (I also switched from German to French bow, mainly because that's what my teacher plays.)
You are lucky that you have already mastered Simandl I & II because you can jump in on any Rabbath book and use it. Me, if I looked at Vol II or III it would look like arabic or some language non-familiar. Way over my head....for now. If I were you, I'd buy what you can of Rabbath's while you can (and while he's still living among us) and learn what he's teaching. If your teacher isn't open to it, maybe try and show him some of Francois' youtube videos, especially the 7 part where he is teaching a class in Germany. Amazing stuff.
François Rabbath at HfM Berlin - 1 - YouTube
For me personally, I can't wait to get through open string exercises!
Here's a video that might help you with your decision. Rabbath method. German bow.
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cheers
ps. wish I could get back to France soon. Would love to get a personal session with the master himself.