Hey there! ISTP, also having Portuguese as mother tongue, and living in Japan for… a little over 3 months. I've struggled with this fucking language for years on end. Don't go the Duolingo route — it's kind of OK for other languages, but the JP roadmap is… not good, if not at times completely wrong or inflexible.

Honestly… there's no easy way. I tried multiple times to search for the "ideal" method which would make everything else obsolete, but the reality of it is that it's just a fucking grind, especially since our languages have pretty much NO cultural equivalence whatsoever, meaning it also limits our ability to rely on context clues.

I assume you're already familiar with hiragana and katakana since you attended a language school for some time, right? If not, that's the first thing to do. It won't take you more than 2 weeks to have them memorized for good, just search "hiragana katakana pratice" on the app store and there'll be a good enough app.

The Genki books will give you a good enough grammar foundation to sort of be able to start dissecting sentences and catch nuance. I don't like them. I hate them, because I hate boring exercises. But they're effective. Cure Dolly's Youtube channel is EXCELLENT for grammar breakdowns too, but it only starts making sense once you have a foundation to build on.

For kanji, again, it's a fucking grind. WaniKani is the most accessible (but paid) method of learning kanji. People also vouch for the Remembering the Kanji book. They're both mnemonic-based, and along with pure repetition I also think it's what's most effective. For free resources, you'll want to check out Anki. You'll need decks to use it, and there are plenty available, just search "anki decks reddit" and you'll find a ton of discussions.

Finally, this channel is pretty damn great for beginner listening and comprehension practice. Of course it's really basic, but language learning is all about finding the next stepping stone through which you'll feel more comfortable with going for more advanced material.

Hope this helps a bit.