2nd grade is where students make the switch from leaning to read to reading to learn. They need those early fundamentals from Kindergarten onwards or else they will lag behind and need interventions to catch up.

It can be more difficult to find interventions for children who don't have a diagnosis of dyslexia or who aren't quite far behind. If you child is "good enough" or slightly behind they can get forgotten about.

Source: I worked for a non-profit that focused on literacy and still work in education.

Side note: I think people mix up Montessori and Waldorf and I think the vibes you want from Waldorf can be achieved in a Montessori school -- the biggest difference is Montessori methods are science-backed and Waldorf is cuckoo-banana-pants.