No matter the level of politeness, learning to speak a new language automatically makes you sound like a child at first Pronunciation-wise、ね。
Also, it’s interesting to hear about you actually starting from a speech-focused course instead of the usual grammar-rules /w reading/writing. Pretty cool! I’m guessing you get a different “feel” for the language via speaking it from the start instead of the mechanical grammar cramming. I’ve never heard of people having problems moving from masu/desu to informal forms, but maybe it’s just something people haven’t deemed worthy to mention. Who knows…!
I do agree with others, preferring desu/masu over informal forms as the starting point… and rather than making it a point to call it “polite form”, I’d stick to calling it “neutral” in this case, as it goes with any and all situations.