FYI greek has a good introductory course by Language Transfer, from which you can graduate to textbooks (available from rutracker) and ChatGPT for practice and OCR. I also maintained decent retention by converting everything I learned into Anki cards. Living in a greek-speaking country helps, but only when you are forced to communicate in EL or when you explicitly ingest new words/phrases from what you read around. Before that, I used Duolingo for the same purpose, had issues keeping the streak up and understanding the material (this was early ChatGPT era, though).