I no longer remember for sure about the earlier courses, but for the three DS has done most recently, I'm sure the initial assessment hasn't been significantly longer than the in-course ones. I want to say we've never seen an assessment longer than 30 questions; I can't say that with 100% certainty, but I am certain that initial assessments for the last 3 courses have been under 35 questions.
Your plan sounds good. FWIW (which may be something, as your DS sounds a lot like mine) we've seen a huge maturity jump in the last couple of years which has made AOPS-type stuff much more usable, and I certainly see that style of thing as the main ingredient in DS's maths at 8. I think the recent ALEKS courses have still added something, basically a low-fuss way to make sure he's seen a coherent bunch of stuff. Neither I nor DS would want to use it continuously, but he may do some more next year.
