Immersion French language learning for an intermediate speaker

I speak French. Often incorrectly, but communicatively. I can certainly make myself understood and I can understand those who care to let me understand. I am pretty terrible at writing and make many grammar mistakes, particularly with regards to verb tenses and gender. My accent is all over the place: Swiss, West African, Central African, American.

I use French for work these days, in sub-Saharan Africa and Haiti, and am learning lots of useful terms like "bailleur de fond," "fosse septique" and "beton armé.". I have some lovely new illustrated construction dictionaries, which are great for vocabulary.

What I think I lack most is the ability to follow French conversations (particularly where more than one person is speaking in a group, I can't shift form one person to another or hear two things at the same time at all) so meetings are really difficult as is watching movies only in French. Speaking on the telephone is torture. Particularly with French speakers from France. Belgians are the easiest for me to understand.

I am finishing up my job (not in a francophone country) and will be moving back to the US soon. I intend to study at the alliance française which will be good, but I feel that without a level of comfort and ease in speaking for pleasure, I will always be just a student of French and not really bilingual. By that I mean that I still revert to English because it is easier or certainly if the French speaking person speaks English better than I speak French. That is natural. But I'd like to get to the point where I am used enough to speaking French that I don't just switch.

I imagine having French-speaking couchsurfers who will continue to speak in French with me, even in the US... There is a line I imagine which I would like to cross it; one more language plateau I'd like to ascend. I am ALMOST there. I can spend a week speaking only French for work and it is exhausting, but good.

I have always wanted to spend a month just studying French and speaking in a francophone environment. I'm approaching fifty years old and can imagine that if I don't do it now, it will never happen. I will become someone who does speak French, but never actually speaks in French...

So, to my question. Where could I go to study French for maybe six or eight hours a day and speak only French the rest of the time? I'm thinking it should be France since they speak fast and grammatically correctly and those are things I find difficult. Much as I love Paris, I wonder if it is less of a good choice because it is so difficult to get Parisians NOT to switch to English these days, just as soon as I open my mouth and am obviously not a native speaker.

On the other hand I am not looking for tourist French or beginning French or even maybe intermediate French. Maybe remedial French,, business french, or technical French. If that were in Paris, I think I could just refuse to speak English if I had to. Technical French for mid-career, semi-bilingual people with a month off... Who teaches that?

what i am finding on the Internet is holiday schools with 15 hours a week of lessons, with nice field trips to lavender farms, and optional cooking lessons. Generally geared to twenty-somethings. That sort of makes Paris more interesting too; not sure I want to spend a month in a village with a dozen people half my age, but also I love Paris!

Advice?

Merci beaucoup en avance.