I'm no polyglot, but talking and reading are very different skills.

My personal working theory:

Native speakers learn to speak first, then they learn to read and write.

From talking with high-level ESOL speakers, those that learned from written material have some very strange patterns of mistakes you can hear in their English - where they've programmed their pronunciation incorrectly very early on with basic mistakes.

Although English is an outlier here. Because English native speakers learn lots of different ways to manage how speaking and spelling are different. Plus because making speaking mistakes after learning words in books is such a social status mistake.

Personally I think if you want to learn a language, the ideal is with someone who corrects all mistakes, as though one is a baby. Although it is difficult to find this even when one can spend months in a foreign country. Not everyone feels comfortable correcting an adult.

The people that have learned English that I have spoken with that are the best all have a common trait: they've lived in a country and they MIMIC. The most amazing was a hippy Japanese guy: I remember one sentence where he shifted in the middle from broad East London to a strong Aussie accent due to where he learned the words.

I learnt conversational Spanish when I was 30. Next attempt will be 2-3 months immersed Korean as a middle aged guy so I'm looking forward to trying out my opinions!