So you're going to send kids to 7 or so hours of being pressured into paying attention to stuff they're generally not that interested in learning, then some amount of time doing mostly make-work activities, and you expect them to be interested in exploring learning opportunities in the minimal remaining bits of time? (And for older grades, they're also getting up well before their bodies want to.)

Would you be able to sustain enough interest to really delve into something when you're that tired out?

Structured activities still work, because the motivation is largely external. I reserve judgement as to how good that is for the kids doing them, but I will agree that it's a workable setup.

IMHO, you can't go halfway. Either you fill up their time with supposedly educational/socializing opportunities, or you give them enough time and space to explore based on intrinsic motivation. With your "unafterschooling", your kids will naturally use it for playtime or zombie screen time. One, or possibly both, of those is likely to be the best possible use of that time, so that's ok; just don't expect them to pick up new skills when you've stacked the odds against it.

(Yes, I'm biased. I'm homeschooling -- not unschooling -- my kids, and there was a major readjustment period to get free of the "school attitude" before things started to click. We couldn't have done that when school was still in their headspace.)