Yes, feel free to join whenever you like!
I should be posting Lesson 3 this week.
I am also reviewing what I learned in university, but it seems that I’ve already forgotten some of the subtle grammar points already (like using ga instead of wa for certain question sentences).
And I am terrible at vocab memorization, so I just note whether the vocab is in WK and which level I’ll learn it in. I don’t try to memorize the vocab, especially when the words are mainly for students.
You can technically start it at any WK level, it just depends on how patient you are with looking up kanji while you study grammar. tbh I found it much easier to study after reaching level 20, but I also was able to study back when I was level 5. I got all the way up to Lesson 8 in Genki and then got distracted with leveling up quickly