> I'm not sure how much interest there is in 80% time for 80% pay
I would take it immediately!
The last time I kept asking about this possibility at job interviews, the best offer I got was 80% time for cca 50% of my market salary. Even then the company felt like they were doing me a huge favor.
> the value of that extra free day is diminished somewhat if it doesn't align with activities you want to participate in or availability of people you want to spend time with
On the other hand, the value is increased if you want to avoid people. For example, shopping is better when there are fewer people in the shop. Generally, less standing in the line anywhere.