I think you have understood well about the difference.
This refers to that you, your friend(s) or you all decided to study at your house every Sunday.
2) 私は毎週、日本語を勉強するようになりました。*Adding a bit more information would be better in my opinion, for example, 毎週火曜日と金曜日に.
It means that you've started studying Japanese regularly every week, however, the reason isn't mentioned yet in this sentence.
I hope this helps you.