IMO, LIngQ is not sufficient for learning German grammar. You need some formal training. We see the same phenomenon with immigrants in Germany. There are countless, who after being in the country for a year or two, attain fluency in the language without formal training, but their grammar is atrocious.
My favorite online source for learning German grammar is „Benjamin der Deutschlehrer“ YouTube channel. Benjamin is a brilliant language teacher. Although he has many free videos on the channel, the real value comes from subscribing. For something like $15/month, you get complete access to all his content, including complete courses for every level from A1 to C2, which would cost you thousands of dollars if you enrolled in a regular German language school.