hi Percy!
i am a full-time work-from-home parent. when everything went sour with DD in Pre-K last year, the wise folks here suggested i think about homeschooling. at first, it seemed impossible to fathom! but i work 11 hours a day! and she's 5!
however, when i totalled up all the actual hours of instruction she had at school (granted, she is smaller than your DS!) it was amazingly slight. your big picture schedule sounds fantastic, and if it seems doable to you - it probably is.
we are only 1.25 days in (literally, we officially started homeschool yesterday) - but we spent the summer getting into the habit of DD5 minding herself during my entire workday. it worked beautifully from Day One. if you'd asked me three months ago if this was possible, i'd probably have laughed till i wet my pants! but the summer routine seems to have set her up well for doing the independent parts of her new school day - and she is LOVING the fact that there's actually work out there that she finds new and interesting.
we do gym from 7.30-8.00 (we go pretty hard - running, biking, scootering, yoga, pilates etc) and then come back to the house for a quick change and then settle down together to work through new/reinforcement academics. an hour later, i turn her loose on some practice-oriented academics which goes until snack at 10.30. then, she has free time until lunch, where we reconvene for another 45 minutes of new/reinforcement academics and we review her independent work from the morning. she then goes off for a second round of independent work until the 2.30 snack. the rest of the day is hers to enjoy. we get outdoors again right at 5 until suppertime. she does her portion of family reading (she's up to 25 pages a night!!) at the start of bedtime and then we read to her until lights-out. she's also got some extra curricular dance (4 classes this year - holy!) and swimming sprinkled in there and on the weekends.
like you, i am also planning to bring in family to help. DH is in charge of Cultural Studies and he's going to take one day off a month to take her around our nearest large city. they'll explore the neighbourhoods, eat food they've never tried before, think about where it comes from, poke around in shops and chat with everyone they meet. they'll sketch in the art gallery and go to the concert hall. DD's grandmother will also take her out to the more obvious museums and things twice a month.
the homeschooling project is obviously very young, but we're off to the most amazing start. it's breathtaking to see her flying already - and it seems a real privilege to actually be there for all the AHA! moments.