Both of my girls, ages 3 and 5, go to a Montessori preschool that goes through Kindergarten. We were sure that DD #1 would go to public school for K, but just recently made the decision for her to do her K year at the Montessori school starting this Fall.
We haven't considered Montessori at the elementary school level, because she'll definitely go to 1rst grade at public school (and her younger sister and brother will likely go to the same Mont. school through K - then go on to public school). However, in an ideal world, I'd have a ton of money and have that as an option to consider. I definitely would think about it.
I've heard stories both ways - that kids transition no problem to public school or to university, and that kids had a terrible time transitioning (specifically thinking of one family whose daughter went to Mont. through 3rd grade, and started 4th grade year in public school - it was painful - but that's just that kid).
IMO there is no way to decide about whether Mont. is good for your kid to do in the upper grades until your kiddo starts at that school. Why not sign kiddo up for preschool there (I've found it a common notion that Mont. is great at least at the preschool level - upper grades, there is more debate).
I don't think you're overthinking it, I just don't think you don't have enough info to make an decisions until your kid starts at a Mont. school somewhere. If it were me, I'd start them in the Mont. school that goes to grade 12 and see how it goes - keep them there, take them out, at least you have the option, and probably enrollment preference, if you decide to stay.
Also, ask about the teacher training - in my preschool search I found many preschool programs that claimed they were Montessori, but really weren't because the teachers hadn't had the extensive training (and continuing education) to really call themselves Montessori teachers. I think some folks use the term loosely.