You want a traditional academic preschool, OP.
Our accredited Montessori was academically-inclined and I loved it. My kids thrived. They knew how to read chapter books by the end of their Kindergarten year (the last year of Montessori preschool) and had completed their thousand chains, could add and subtract large numbers and were exposed to the concept of multiplication and division. They could write phonetically.
Montessori is supposed to be very structured, but within that structure the teacher allows the child to start with whatever work they choose. At some point during the day or week they need to complete all their work, corresponding to their age and competence. The mixed age works well, because children can work at their own pace. The older kids act as role models for the younger, which is very good for them, and the younger kid learn for the older ones. The environment is supposed to be calm and relaxed. There is also time to goof off and play outside, or with toys. The work materials are kept as work, not play, even though they are very appealing.
Be aware that the Montessori name is not protected and therefore you need to check that each "Montessori" you're looking at is property accredited with Montessori-trained teachers and aides. This may be why you started with the wrong idea
Also remember that ultimately it's not the method that matters most, it's the teacher! An excellent teacher will find ways of increasing your child's critical thinking abilities, communication skills, resilience and initiative, regardless of the activity. This is what made our Montessori so good: the teachers were excellent.