Monthly Spreads
There is a temptation to treat monthly spreads as a checkbox to clear before moving on to the more interesting parts of bullet journaling. That is exactly backwards. Monthly Spreads is where a real understanding of the craft starts to develop, because the small choices you make about monthly spreads reflect almost everything you have learned so far. People who skip monthly spreads hit a ceiling within a year and cannot see why.
The other way round: time spent on monthly spreads pays compound interest. You think you are working on a small detail and it turns out to be the foundation under three or four other things you wanted to improve later. If you are choosing what to focus on next, choose monthly spreads more often than you think you should.
A final note. The aim of bullet journaling is not to look like someone who does bullet journaling. It is to enjoy the doing — the slow build of competence, the small surprises, the days when something just works. Keep the gear modest, keep the schedule sustainable, and pay attention to choosing a notebook. Most of what is good about the hobby will arrive on its own.