I had your dilemma a bit. My kids are now 9, 7 and 6 in public school after homeschooling, I'll share my experience...
My ex and I are extremely artsy and think WAY OUTSIDE the box. I'm a painter and he's a heavy metal musician. We're both self-driven, hard workers who had to build our own careers AND had to conform to many jobs before building our own careers. We're both products of good old-fashioned public school and straight-laced, non-creative parents. We had kids late.
Many people who knew me well recommended Montessori and Waldorf schools since we're "creative types". I was skeptical of all that play but I went to tour a Quaker school near us (similar to those). The principal showed me all the fancy, crafty spaces and how fun everything was, and bragged about how child-centered and individualized all the teaching was. I don't know. My gut was screaming to get out of there. Our house is like one big loopy doopy art center. All my kids know is art, music, playing...I needed someone to CRACK THE WHIP on all the boring, mundane, sitting still stuff, because self-discipline, conforming, and rote learning are important skills too. (I think I was French in another life). HOWEVER, I was aware of the deficient quality of public education too, so for a few years I homeschooled in the Classical style with very advanced materials. The kids thrived on the structure and formal work and entered public school way ahead on all their levels a little over a year ago.
The pros and cons of public school are as I thought they would be: Good structure, socialization (thought we had plenty of socialization homechooling too), following an institutional program, conformity...blah blah. Cons: Work is way too easy. Lots of time is wasted waiting for behavior issues in classes, and the classes are test-geared and catered to medium/slow students. We live in a poor town, so the average child has not been pushed at home, so the levels are lower than if we were in a more educated area. My kids are no longer way ahead, they went down to the levels of their classes of course over the year (on paper, though they had untestable assets from home) because they only learned what was taught there.
So. I still wouldn't switch to an expensive alternative school, because our home life is so creativity based, but I do supplement at home with classic literature, cursive, advanced history, and lots of hands-on play and learning to try to counter-balance the long school days.
The kids love school and learning though so far, and that's what really matters. The experience can be enriched at home whatever the base...
I kept learning very loose through kindergarten and didn't start advanced writing stuff and grammar/ math til first grade in keeping with European countries I admire. Mainly reading aloud and hands-on learning for kindergarten...my younger kids listened to my older kids more formal lessons, but mostly they played. So if you did want to keep him Waldorf for kindergarten, he might be behind at first on sitting still a long time and worksheet stuff if you move him to public first grade, but he'll catch on and no harm done. My son entered kindergarten for the last 6 weeks of school and he was behind his class on worksheet writing even though he knew lots of stuff his classmates didn't. He caught up quick and enjoyed the challenge though. In first grade he was way ahead because of his homeschooling plus acquiring public school skills...again though, he leveled out by end of year because there was no way to move ahead of what his class was learning. He got straight As and highest marks though on that material.
As for longer term...I don't know how Waldorf kids perform in adult years compared to public school educated kids. So many variables....