do I have to buy all the courses to progress through my certifications?
IMO, most of those certifications are not worth "progressing through" and most C-cards have no practical utility, in the sense that nobody will ever ask for them. The only thing that matters is to have someone get you out of your comfort zone and help you find the things you could improve on. If the instructor did not make that effort, I would try to initiate that conversation before signing up for the next class. Personally, I have taken little out of formal courses. I have not learned anything about navigation until I started diving solo, and had to actually look around, and pay some attention. Like others in this thread, I also find it difficult to follow compass heading in current. As far as I can tell, navigation is all about identifying and remembering waypoints, which for me at least, at my present skill level, translates to gradually getting familiar with the site by progressively exploring it.