My general advice is to find a course that lets you hear and speak the language. Also, if you’re planning on going into linguistics, it’s worth working with a descriptive grammar of the language with many examples in context. Ultimately, though, nothing beats interacting with native speakers.

More specifically, I studied Swahili for a book I wrote (Native Grammar: How Languages Work). The pronunciation (and writing) are easy to pick up, although there are a few tricky consonant sounds. Fortunately, as you mention, Swahili is a "koine" and got rid of Bantu tones.

Swahili grammar, on the other hand, works quite differently than English. You’ll have to do a bit of what I call "linguistic problem solving" to fit the pieces together. Agglutination and agreement are the keys. But stick with it and, soon enough, you’ll have mastered all those pesky noun classes and verb affixes.

You can learn all that grammar in-depth by comparing real-life Swahili examples and then practicing yourself. I wouldn’t pour over too many rules without putting them to practice first.

I’m glad to hear you love languages – keep at it! Make mistakes and struggle early on, and you’ll be a pro in no time.