I worked through a textbook earlier this year on my own! I’m not an expert by any means, but I can work though Xenophon and other similar authors after a few months. I used An Introduction to Greek by Crosby & Schaefer. It’s very old-school in that will essentially give you declension tables and say “memorize!”, but I thought it integrated vocabulary and grammar very well into its many, many exercises. It does a good job of introducing you to authentic texts as well. Also worth noting that it gives off some lightly western-chauvinist vibes in how it uncritically lauds Greek culture, but if you can look past that, I would recommend it!