Building my own toolset and curriculum is exactly what I want to avoid.

I get what you’re saying. It is a really weird phenomenon that:

1) Everyone has basically the same problem (learning a language).

2) Many people are building their own tools.

3) Most people are dissatisfied with the tools available.

I have a hard time believing that there is no more efficient way possible than conversing with native speakers as one commenter seemed to suggest.

I also don’t believe the “procrastination” suggestion. My girlfriend has worked through the entire path on Duolingo Spanish. She can’t speak Spanish.

In my personal experience, the easiest way to learn a second language is to immerse yourself in a particular country/culture and interact with the locals, get a job, etc. Live there for at least a year. It worked for me, I learned English and it worked for my wife, she learned French.

Are you French and your wife English-speaking? That’s what the OP article mentioned as “école horizontale”. The author lived in France for years but her family spoke English at home and thus her French proficiency never got great.

In my experience living abroad, the fact that so many people speak pretty good English is both a blessing and a curse. We could have a conversation in French but it’d be pretty boring. So my friends and I always speak in English but I don’t get better at French.