The Fenix school program sounds good ... if it's still in business.
Immersion is helpful for sure but you also need, IMO, someone to sit you down and teach all the basics ... correctly!. Verb forms, vocabulary, accent.
To really learn something requires pretty hard work on students part.
Some people are natural linguists, pick up accents and such quickly, others struggle. I got the accent part but getting into Pluperfect subjunctive verb forms were a bit of a challenge ... but most forms are fairly simple. Getting taught ... then USING what you've just learned is GOOD.
But once you get past basics ... then immersion teaching will really help ... as you will learn common phrases in use ... and learn lots about the two forms of the verbs TO BE ... Ser and Estar. Those are key to learning.
I would continue to look around and research. I've heard of schools in Mexico City and also in San Cristobal de las Casas. (nice town)
Here on HUBB, most travelers end up doing Spanish class in Guatemala, specifically Antigua. Many many schools with many featuring semi unqualified teachers.
A "real" Spanish teacher should have university level training, IMO, but plenty
claim expertise and just wing it. Also Spanish schools all around Lake Atitlan (where I lived for 2 years).
I would ask and look carefully before committing and handing over money.
!que le via muy bien!