Yes, you will receive all the materials at once for K, and you should be able to just whiz through assessments if that's the route they want you to take (that is, if they don't want to assess him into a higher level immediately). We just whizzed through the K math assessments, most of them. There were some random things we needed to cover with her, and then the last month or two of lessons were actually 'newer' learning for her.
I think, from reading descriptions, that the state impacts the flexibility of K12 experiences for people; and then that is magnified (or minimized) by the teacher. Our teacher has been strong on pushing moving to the assessments if the kiddo has it figured out, alternative methods or substituting coursework etc. For instance, we do have it worked out so that Ina can do music lessons privately and cancel out the K12 music lessons, next year. She knows that we travel and some days we are just going to mark hours and attendance, but won't have done any of the coursework; and that other days, we may really load up -- and that sometimes we'll work on the weekend, or etc. This is WY's first year, and so far there isn't a lot of state micromanaging, and she is effusive in complimenting us on how we're doing and etc. So -- we actually really love our teacher; she was very helpful when Ina was struggling with Phonics, too. She gave us good advice.
I honestly think one reason we're staying in k12 is because I don't want to let her down.

We do like having a 'canned' curriculum. I like not having to think about it, and being able to just expand it if I want to (we expanding things on slavery, for instance, recently). From what I've heard about the local schools, I am certain that K12 has Ina ahead of her peers in math, science, history, and probably even reading/phonics. Her handwriting is still a little wonky but I think that's normal too. And we'll work on it this summer.

Our local schools are actually supposedly well ranked for our state ....
I do think that it will become more complex when dd2 starts school, too. Certainly more time on the computer!!
And, I still think we will likely pull out once the state tests start (3rd grade). We might go ahead and do it one year, just to see if it's as awfully focused on the tests as I fear -- but if it is, we'll be outta there for sure.
I feel like the science, history, and math curriculum are really good. A lot of the science was stuff Ina already knew (heck, most of it) - she could easily have worked a level or two up. But I think most K kids would enjoy it and do well with it. I am not a huge fan of the Phonics (too dry, difficult for Ina to connect to it). The Language has been _OK_ (some really fun stories/books, I loved Ox Cart Man) but some were more scary than Ina could handle, and I didn't like all the princessy stories at the beginning of the year .... I abhor the music program and we are, indeed, just marking attendance for that


We do try to find music to enjoy otherwise, but just couldn't get through the music lessons. Maybe they improve when kiddos are older, or when the parent has a teaspoon of musical talent herself, I don't know.

I am divided about the Art section, I rather feel like sometimes it's too heavy-handed about telling kids how to draw things (like, how to draw a NOSE?!) -- I am artistically talented, and I figured that sort of thing out on my own. But, then again, maybe this level of direction would have been even better for me? (Rather than stifling?) -- or, maybe this level of direction is good for kids who are needing to practice and learn technique?? I don't know .....
So, basically, I like most of the materials, and the support (and lack thereof) that I've received. And I feel now like I know enough about how to balance this, and personalize it, that I'm looking forward to next year. Dh and I were talking the other day (about this thread/discussion, in part) -- and he and I agree, that I need the structured deadlines to keep on track with what dd1 should be learning. I guess we're neither of us really "unschoolers" at heart, and Ina is responding well to this program .... I think I could probably fudge in my own curriculum for science, history, art, and follow a curriculum for math; but I don't know if I could make myself work through challenges with phonics/reading like we had this year. The timeline definitely kept me working through that situation. Maybe in a few more years, I'll feel more confident about being 100% ME for the curriculum. From the beginning, the thing that intimidated me the most was the idea of teaching phonics/reading.