Preface, I went to a Dutch Montessori school. Curriculum is probably very different. Also school starts from the age of 4/5 and goes till 12/13. (group 1-8)

Monstessori is definitely not for everyone, I really enjoyed my time there (not counting the traumatic experiences caused by classmates). I definitely feel like the teachers there can give you a more personalized way of learning which just isn't possible on standard schools. The national curriculum did not challenge me enough and had me really struggling to do any work, eventually my teachers allowed me more time on the computer if I did my work and that worked really well for me.

For transitioning to high school, at first I quite enjoyed it as it was all new but after a while I just lost all motivation. Every meeting with my parents and the school basically came down to "We really think monkeyguy can do better, but he just doesn't seem interested". It sucked, I barely did anything and I barely passed each year. Eventually I graduated and the same thing happened with college/uni. Work life has been great however.

I'm not going to recommend anything however, I do not know your children as well as you do.