10:56 AM, Wednesday August 11th 2021
So short answer: No, it is not counter-productive.
First of all i think you have a good approach on draw-a-box, i liked how you said that you want to finnish it in a marathon. This is a very healthy approach.
In the end its all a matter of what situation you are in. If you were trying do to as many courses in a shortest period of time possible, that would be counter productive. This way you would try to gain as many informations and knowledge about drawing as you can, but things just dont work like that. In the end it all runs down to you having to draw a lot, courses will only point you into a certain direction or path so to say.
Draw-A-Box is not supposed to be some kind of prison or dark room you need to stay in till you finnish every single last lesson. Furtermore its no "betrayel" if you want to gather informations about other topics such as anatomy. Im just gonna say it as it is, as the same name already suggets "draw-a-box" is no complete guide to every single field of the spectrum of art, that means that you will sooner or later will need to do other courses if you want to gather more skills. Draw-A-Box strengthens fundamentals, which will always be very very important. You learn how to construct primitive form and how to put them into perspective. Essentially things will always run down to these fundamentals, thats why it is so important. You will soon realize how anatomy becomes easier for you because you are able to abstract those organic shapes into simplier ones. Thats why i think that putting other courses in addition to draw-a-box might help you actually apply those things you learned in draw-a-box to other topics.
Now to still warn you and not let you dive into some rampage of doing new courses every week. Just dont overgrind. Remember the 50/50 rule uncomfortable talked about. Always keep yourself a piece of fun that you can come back too. Im not saying that courses are always super dull and stuff but i think you already know what i mean. If you expect too much of you, you will just burn-out and stop drawing. We all dont want that.