Oh, one more thing. There are also "Grammar Workbooks" which consist of hundreds of pages worth of drills.

If you are a nerd, these hundreds of pages of exercises might be more important than reference material. Buying a workbook so that you can DIRECTLY write on the pages and try immediately is also helpful.

Grammatik Aktiv by Cornelsen covers A1 through B1 pencil-and-paper drills. Very dry stuff but it kind of works...

You need a separate textbook to know what order to learn things (it sounds like your Goethe Institute course covers this). You need additional reference (Basic German: A Grammar and Workbook covers this, a 2nd clear perspective focusing on grammar). Finally you'll find that various bits of your speaking + writing skills suck.

Using Grammatik Aktiv exercises to drill on your weaknesses just makes sense. Maybe an intensive would try to complete the whole book but uhhhhh.... self study means you get to choose when you're done with exercises lol. Do as much as you see fit.

Grammatik Aktiv is however, 100% in German. You probably need to wait until you are A1+ before you buy Grammatik Aktiv, if only so you have enough vocabulary to even figure out what the drills are asking of you.

Maybe your A1 goal should be to learn enough German so that you can start Grammatik Aktiv, lol.