That said, my impression is getting to functional in a language quickly requires referencing a source language that is fully understandable by the user to build vocab and comprehension - ie. explaining a new concept in the target language using the target language for a B1 student is going to be inefficient and not expressive enough. Otherwise you're fortifying what you already know vs. actually building more knowledge. Things like comprehensible input are great but seemingly more indirect and less efficient.
If you have an option to get from zero to B2 fairly quickly, you are functional enough in the target language to use a myriad of options to fluency, including doing nothing other than conversing with others.