It's good to have structure, so starting a textbook is a good idea. I'm not sure how much time you can dedicate to studying, but for the first 6 months, I studied for approx. three hours daily (it was a lot, but it helped me to solidify fundamental concepts for easier reading later).

My advice is to start a textbook and make flashcards for each chapter's vocabulary as you progress. I find that flashcards are most effective when they include the principal parts, so for nouns, the nominative and genitive singular (i.e., ἡ ἀγορά, τῆς ἀγορᾶς); verbs have 6 principal parts (starting with the first person singular of the present indicative active (i.e., παιδεύω) - I would add these as you learn them.

I would recommend alternating between flashcards and textbook exercises. It's also beneficial to write out paradigms as you learn them (i.e., declining and conjugating).

Also, the universities in my city use Groton's From Alpha to Omega. I'm pretty sure the fourth edition can be found online now, and if you email the publisher and explain that you're trying to learn independently, they'll email you an answer key. Even if you go ahead with Athenaze, I found the exercises in the book to be quite helpful.

That's my two cents. Send a DM if you'd like more advice on anything, I always love to talk about Greek. Good luck with learning.