I don't think 6h/day of work is particularly unreasonable, if you can avoid losing blocks of time to useless activities (unneeded meetings, or waiting for people).
If you're remote, and only clock-in when actually working, spending 3-4h/day working productively can easily outdo someone spending 10h/day in an office. I've worked in places where my ultimate client billable rate was obscene, and tracked to the 6min or 15min interval, but we also had "house time" for other projects. A day where you spent 3h directly working for the customer and nothing else was perfectly fine (especially if you blocked the entire day out with "need to be available for client" so no one else could interrupt you.)
Spending the downtime doing actually interesting research (not so much 20% time as 50% time) was awesome.
If you're remote, and only clock-in when actually working, spending 3-4h/day working productively can easily outdo someone spending 10h/day in an office. I've worked in places where my ultimate client billable rate was obscene, and tracked to the 6min or 15min interval, but we also had "house time" for other projects. A day where you spent 3h directly working for the customer and nothing else was perfectly fine (especially if you blocked the entire day out with "need to be available for client" so no one else could interrupt you.)
Spending the downtime doing actually interesting research (not so much 20% time as 50% time) was awesome.