While doing numeric "challenges" like that (draw 100 heads, draw 100 hands, that sort of thing) can definitely be useful, for beginners it really does help to have the benefit of an actual structured course to help introduce them to the concepts of a given topic and how they might go about practicing them. Diving right into it without that guidance means that you're effectively serving as instructor yourself. That's a position one can take more easily when they're somewhat further along, but doing so earlier on has a tendency to cause students to focus just on how those studies/drawings come out superficially, as that is the only way they are able to judge the work - which can cause them to miss the underlying issues that they actually need to work on. More than that, students in that position can hyperfocus on the superficial issues and actually end up missing what might really be holding them up.

That's not to say self-teaching isn't viable, it's just an up hill climb for the very reason that the student would be attempting to teach them something they don't know, with books and one-sided tutorials not being able to catch misunderstandings.

Speaking of misunderstandings, you mentioned that you were practicing your fundamentals "for the 50% rule". You may be referring to the "study" half of the 50% rule, which includes Drawabox, but generally when students talk about the 50% rule, they're talking about the other half. Just to be clear, studies, tutorials, courses, etc. all belong to the same 50% as Drawabox's own assignments and homework. The other half is strictly reserved for drawing for the sake of drawing, without the focus being on actually learning or improving your technical skillset.