How do you generally categorise beginners, intermediate and advanced singers? And generally what is the process of vocal development? Would be good to kind of have a roadmap to know where you are and where you're trying to go or maybe that's not what it's about?

This is how I'd think of those categories (for the purposes of this site):

Beginner - discovering + learning the elements of vocal control

Intermediate - good (but still developing) vocal control

Advanced - mastery of vocal control

Registration, vowel modification, agility, etc can all go under that umbrella of "vocal control", in my opinion. 

Honestly, it's a balance. It's good to have goals, but it's also good to enjoy the journey of singing and see it as an art, not a sport. Ultimately, it's up to you! But I personally fear that too much analysis can take the joy out of singing.