So which differences do you see between your understanding of the Great Work and the Rosicrucian's (and offshoots like Thelema)?
I could maybe write a short compendium of essays on the subject with each one varying in their degree of quality but I think I'll just give you a fairly illustrative example.
I knew a few Adeptus Majors and Magisters from the O.T.O. and there was this one private chatgroup I was in specifically for discussing Thelema and related occult movements. Not everyone there was a Thelemite. In particular, there was this one guy (we can call him Norton) who refused to label himself anything (although I'm pretty sure he was a Rosicrucian) and we would get into these very lengthy debates. We never really saw eye-to-eye on anything.
Some of the major disagreements we had:
-Norton was convinced that Enochian was essentially the source code of the universe and that scrying the Enochian Aethyrs was the most important and authentic way of undergoing the Magnum Opus. Even when I believed in Enochian magic, I still thought that the system was incomplete and too dangerous to work with and I never bought that it was the "true" variation of Celestial out of the countless versions that popped up before and after John Dee.
-Norton stressed that, at the end of the Great Work, one could literally transmute matter and that the true alchemists who hold this power hide themselves in plain sight, walking among the uninitiated like ordinary people. I never really bought that.
-Norton believed that virtually every ritual and ceremony had to be performed literally and physically to the letter. I suspect that he actually had the proper medallions made of the proper metals at the specific astrological times and he explained to me how he got his lion's pelt and a white robe spun by the hand of a virgin. He believed in the importance of these elements so much that he said that real magic could never be performed without them. I never really bothered with all of that because I'm too poor, lol, but all he ever claimed to achieve through it was synchronous effects, anyway
-Norton also believed that these ceremonies were absolutely necessary for the Great Work, and that even the most rigid of the modern institutions claiming to be the A.'.A.'. have strayed from their original vision of creating the true Holy Catholic Church and purifying the apostolic line of succession, that the Magnum Opus was ultimately one's quest to achieve a degree of sainthood that conveyed nigh-omnipotence and physical immortality. I always thought that the Great Work was an internal and subtle process which was mostly about achieving a degree of wisdom in life.
-Norton believed that morality is written into the fabric of the universe and was, well, kind of alt-right in his understanding of this morality (which is a polite way of saying he was queerphobic and still perpetuated the Red Scare). He believed that only institutions with a direct and traceable lineage to holy men had the authority to speak on these matters. Only they had the wisdom and the initiated power to know the best rituals for any new followers and they held the ultimate say in matters of morality. I was an anarchist and going through my anti-organized religion phase at the time but I still wholly believe that proper study and an earnest desire to connect with whatever might be out there is all you should need if there is something out there. I doubt that it would filter itself through unnecessary middle-men. It's one of the reasons that I still don't believe in prophets or apostolic succession.
We were very night-and-day until he eventually left the chatroom because he was upset that everyone there was, in his perception, so deeply misguided that they were doomed to be eternal slaves of Choronzon who he incessantly referred to as "Mr. C."